Ingredients
Method
Make the Funfetti Sheet Cake
- Preheat the oven to 325°F (163°C). Prepare a 9x13 metal baking pan by spraying the sides with baking spray and lining the bottom with parchment paper.
- Sift the cake flour, then measure it by spooning and leveling into a measuring cup. Whisk the sifted flour, baking powder, baking soda, and salt together in a medium bowl and set aside.
- Combine butter and sugar in a stand mixer bowl with a paddle attachment. Beat on medium-high speed for 4 minutes until light and fluffy, scraping the bowl once. Add eggs one at a time, mixing for 10-15 seconds after each, then mix in vanilla and sour cream on high speed for one minute.
- Turn off the mixer and add the dry ingredients. Mix on low speed until just combined, then pour in the milk in a steady stream and mix until incorporated. Scrape the bowl and stir by hand to ensure no lumps remain.
- Toss rainbow sprinkles with 1 teaspoon of flour and fold them into the batter. Pour the batter into the pan and bake for 30-35 minutes until the cake springs back and a toothpick comes out clean. Cool completely in the pan.
Make the Vanilla Buttercream
- Cream the butter in a stand mixer with a paddle attachment on medium-high speed for about 7 minutes until it is pale and creamy.
- Add powdered sugar gradually, mixing on low speed and scraping the bowl between additions until fully incorporated.
- Add vanilla, milk, salt, and optional pink food coloring, then mix on low speed for one minute until fully combined.
Assembly
- Frost the cooled cake with half of the buttercream, using an angled spatula to create texture. Garnish with sprinkles, then pipe a star border around the edges using the remaining buttercream and a piping bag with a star tip. Finish with additional sprinkles.
Notes
*DIY Cake Flour Recipe: To make your own cake flour, spoon and level one cup of all-purpose flour and remove 2 Tbsp. Add 2 Tbsp of cornstarch. Repeat per the amount of cake flour you need, then sift the flour and cornstarch mixture 4 times (don’t skip that step!). After sifting, spoon and level to re-measure the amount of cake flour you need. Keep in mind that since store bought cake flour weighs less than homemade cake flour, 2 1/2 cups of homemade cake flour will weigh closer to 331g instead of 265g.
*9×13 Baking Dish: I used a metal pan for this sheet cake, but a ceramic or glass baking dish will work just fine if you add 10-15 minutes to the baking time.
*Rainbow Sprinkles: Be sure to use the elongated rainbow sprinkles (jimmies) and check the ingredients label for “carnauba wax” and “confectioner’s glaze”. These two ingredients will ensure the sprinkles do not bleed color when folded into the batter.
Make Ahead Tips:
- The funfetti sheet cake can be made ahead and stored at room temperature, covered tightly in plastic wrap, for up to two days. Alternatively, you can cover with plastic wrap and store in the freezer for up to two months before thawing to room temperature.
- The vanilla buttercream can be made ahead and stored in an airtight container at room temperature for up to 1 day. Alternatively, you can store it in an airtight container in the refrigerator for up to two weeks. When you’re ready to use it, bring it back to room temperature and rewhip with your stand mixer to bring back to frosting consistency.
- The frosted sheet cake can be stored at room temperature, covered with plastic wrap or aluminum foil, for up to five days.