two 8-inch greaseproof cake boards; a piping bag fitted with an open-star tip, such as Wilton 1M
For the confetti cake: Preheat the oven to 350 degrees F. Line three 8-inch round pans with parchment rounds, then grease with nonstick cooking spray.
Mix together the flour, granulated sugar, baking powder and salt in a stand mixer fitted with the paddle until fully combined. Mix chunks of butter slowly into the dry mixture with the mixer running on low speed until no large chunks of butter remain and the mixture becomes crumbly. Pour in the egg whites and mix on low until just incorporated. Mix in the buttermilk.
Add the vanilla extract, almond extract and oil, and mix on low speed until fully incorporated. Scrape down the sides of the bowl as needed.
Fold the sprinkles into the cake batter using a rubber spatula, just until they're evenly distributed. Divide the batter evenly among the prepared cake pans.
Bake, rotating and switching the positions of the baking pans halfway through, until a toothpick inserted in the center of a cake comes out clean, 34 to 36 minutes. Allow the pans to cool on a rack for 10 minutes, then run an offset spatula around the perimeters of the pans to separate the cakes from the pans.
Place the cake layers in their pans in the freezer until fully cooled, 45 minutes to 1 hour. Carefully flip the pans and remove the layers from the pans.
Use a serrated knife to level the tops of the layers. Set aside.
For the vanilla buttercream: Meanwhile, beat the butter in a stand mixer fitted with the paddle attachment on medium speed until smooth, about 30 seconds. Turn the mixer to low speed and slowly add the powdered sugar 1 cup at a time, alternating with small splashes of cream. Continue to beat until fully mixed, then add the vanilla and salt and beat until the ingredients are fully incorporated and the desired consistency is reached. (If the frosting is too thick, add more cream 1 tablespoon at a time. If the frosting is too thin, add more powdered sugar 1/4 cup at a time.)
Stack the first cake layer on a greaseproof cake board, using a dab of frosting to help it stick. Add an even layer of buttercream to the top of the layer, then continue to stack and frost the layers in the same manner.
Apply a thin coat of frosting around the sides of the cake to fully cover the cake layers. Add as little frosting as possible to the top of the cake, to make it easier to handle during the sprinkle roll. Smooth the sides using a bench scraper, then chill the cake until the frosting is firm to the touch, 10 minutes in the fridge or 5 minutes in the freezer.
Add a second, thicker layer of frosting to the sides of the cake, and smooth using a bench scraper. Place a second cake board on top of the cake.
Fill a baking pan with an even layer of the rainbow sprinkles, then carefully pick up the cake and roll it in the sprinkles. Place the remaining frosting in a piping bag fit with an open-star cake decorating tip, such as a Wilton 1M tip, and pipe swirls around the top of the cake.