Party-Size Vanilla Birthday Sheet Cake Recipe

This party-size vanilla sheet cake is inspired by the nostalgic Costco birthday cake. It’s built from two large, thin vanilla cake layers (optional sprinkles), a silky vanilla custard cream filling, and smooth vanilla Swiss meringue buttercream for the outside. The format — baked in two half-sheet pans — makes it easy to fill, frost, slice, and serve for a crowd.

The pastry-style custard filling gives the cake a classic bakery feel and balances the buttercream, creating more interest than a cake filled with frosting alone. This is the kind of cake that works for birthdays, graduations, holidays, or any event where you need a large, homemade-feeling vanilla cake. And it’s highly customizable to match your taste or theme.

The Details

1. Vanilla Cake – This recipe doubles a standard 6-inch vanilla cake base so it spreads thin and even across a half-sheet pan (roughly 13×18 inches). Use or omit sprinkles depending on preference.
2. Vanilla Custard Cream – The filling is a lighter take on pastry cream, similar to a diplomat cream: pastry cream folded with whipped cream to stay rich but not overly heavy.
3. Vanilla Buttercream – The exterior is vanilla Swiss meringue buttercream for a silky, less-sweet finish. If you prefer a sweeter, simpler option, American buttercream will also work.

Vanilla Cake

1. Cream butter and sugar. Beat together with a whisk attachment for 1–2 minutes until light.
2. Add oil and continue beating for another 1–2 minutes.
3. Add eggs one at a time, beating 30–60 seconds between each. Stir in sour cream and vanilla just until combined.
4. Alternate in dry ingredients and milk: Add half of the dry mix, then milk, then the remaining dry ingredients. Mix only until the last streak of flour disappears.
5. Fold in sprinkles if using, mixing just until distributed.
6. Divide the batter between two half-sheet pans (about 44–45 ounces of batter per pan).
7. Bake 15–25 minutes at 350°F (177°C) until golden and the center springs back. Cool completely before removing from pans.

Sprinkle Options

For mixing into batter, choose soft jimmies or confetti sprinkles that won’t dissolve into the batter and bleed color. Avoid nonpareils for batter mixing; they can bleed or create uneven colors if over-mixed. Any sprinkles work fine as a topping.

Vanilla Buttercream

Note: The recipe uses more egg whites for the buttercream than the yolks used in the filling. You can make 75% of the Swiss meringue recipe (using 6 egg whites) to lightly frost the cake if you want to avoid leftovers; it will be enough for a smooth coat but not for elaborate piping.

1. Dissolve sugar and whites: Whisk egg whites with granulated sugar over a double boiler until the sugar dissolves and the mixture reaches about 170°F (77°C).
2. Whip into meringue: Beat the warmed mixture on high until glossy, marshmallowy, and no longer hot.
3. Add butter in chunks: Add room-temperature butter a little at a time; the mixture may look loose at first but will emulsify with continued beating.
4. Finish with vanilla and salt: Mix just until combined and the buttercream is smooth.

Vanilla Custard Cream

Make the pastry cream after baking the cakes so it has time to cool. Fold in whipped cream right before assembling to keep it light and spreadable.

1. Heat the milk until steaming or just simmering.
2. Whisk eggs, sugar, cornstarch, and salt together until smooth.
3. Temper the eggs: Gradually whisk hot milk into the egg mixture, then return everything to the saucepan.
4. Cook until thickened: Stir continuously until the cream thickens, then whisk another 30–60 seconds.
5. Finish off heat: Remove from heat, whisk smooth, then add butter and vanilla. The cream may separate briefly but will come together as you whisk.
6. Cool covered: Press plastic wrap to the surface and cool to room temperature; refrigerate if needed, whisking gently before folding in whipped cream if it firms up.
7. Whip and fold: Whip heavy cream to stiff peaks and fold into the pastry cream until combined.

Assemble

1. Place the first cake layer on a flat cake board or serving surface.
2. Pipe a buttercream border around the edge to form a dam.
3. Fill the center with the vanilla custard cream.
4. Set the second layer on top and chill briefly if needed, then frost the outside with the remaining buttercream. If the top layer feels fragile, refrigerate 20–30 minutes before frosting the exterior.

How to Transfer Large Layers

For the bottom layer, flip it onto a large cutting board or flat surface and then flip it back onto the serving board. Use a large cake board if you have one — standard cutting boards can be too small. For the top layer, slicing it in half and transferring the halves separately makes it easier and safer, since the cake will be sliced for serving anyway.

Pan Sizes & Servings

This recipe uses two half-sheet pans (13×18 inches; interior dimensions roughly 12×17 inches) and yields about 48 small-to-medium slices. Any pan with similar surface area will work; avoid making the layers significantly thicker or thinner than shown. Thinner layers are fragile and thicker layers compress the custard filling.

You can halve the recipe and bake it in a single half-sheet pan, slice that baked layer in half, and make a two-layer cake that serves about 24 slices.

How to Customize a Party Cake

Decorate to suit your occasion. The Swiss meringue or American buttercream both make excellent piping and finishing frostings; if you skip the custard, double the frosting quantities to fill the center. You can also adapt any 6-inch cake flavor recipe to this party-size format — chocolate, marble, chai, Fruity Pebbles, Oreo, Nutella, and more all translate nicely when doubled and baked in sheet pans.

Serving & Storing

Because the filling contains custard, store the cake covered in the refrigerator. Bring it to room temperature 30–60 minutes before serving so the buttercream and cake soften slightly, but slice while still somewhat firm for the cleanest cuts. The assembled cake is best eaten within a couple of days; for longer storage freeze slices or leftover portions as soon as possible.

Thank you for trusting this recipe for your celebration. If you make it, rate or leave a review — and enjoy your baking!

Love, B