Preheat oven to 350°F (177°C). Grease three 8-inch or 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

Whisk the cake flour, baking powder, baking soda, and salt together in a large bowl. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed for 5 full minutes until completely creamed together and fluffy. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the oil and beat for 1 minute until combined. Add the eggs, egg whites, and vanilla extract and beat on medium-high speed until combined. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed.

Add dry ingredients and buttermilk in three additions, mixing after each addition just until incorporated. Do not over-mix this batter. The batter will be velvety and thick. Gently fold in the sprinkles. Pour and spread the batter evenly into each cake pan.

Bake for around 25–26 minutes or until the cakes are baked through. (8-inch cakes may take about 27–28 minutes.) Tent the cakes with foil after about 15 minutes to prevent the sides and top from over-browning. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done.

Allow cakes to cool in the pans set on a wire rack for 20 minutes. Run a knife around the edges and then remove cakes from pans. Place cakes directly on wire racks to cool completely. The cakes must be completely cool before frosting and assembling.

In a large bowl using a handheld mixer or stand mixer fitted with a whisk or paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cream, vanilla extract, and salt. Beat on low speed for 30 seconds and then switch to high speed and beat for 2 minutes. Add more confectioners’ sugar if frosting seems too thin; more cream if frosting is too thick; or an extra pinch of salt if frosting is too sweet.

First, using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Place one cake layer on your cake stand, cake turntable, or serving plate. Using a flat icing spatula or small offset spatula, evenly cover the top with about 1 cup (about 250g) of frosting. Repeat with second cake layer, 1 more cup of frosting, and then the top cake layer. Spread a thin layer of frosting on the top and around the sides as a crumb coat. Run a bench scraper around the cake to smooth out crumb coat. Chill uncovered in the refrigerator for at least 30 minutes and up to 3 hours to set the crumb coat.

Spread the remaining frosting all over the top and sides. Garnish with extra sprinkles, if desired.

Serve cake immediately, or if you need to store it for a few hours, place it in a cake carrier at room temperature for up to 4 hours. For longer storage, refrigerate covered cake for up to 1 day. Cake is best served at room temperature, though, so if it’s been refrigerated, let it sit out at room temperature for 30 minutes before serving.

Cover leftover cake tightly and store it in the refrigerator for 5 days. Again, I like using a cake carrier for storing and transporting.