A triple layer 4-inch vanilla cake recipe that's tasty and perfectly sized for a smash cake. Top with vanilla buttercream for a classic flavor combination.
Preheat the oven to 350°F. Prepare three 4-inch cake pans by spraying the sides and bottom with cooking spray (Baker's Joy is my favorite). Sift the cake flour and then measure by spooning and leveling it in your measuring cup. Add the cake flour, baking powder, baking soda, and salt into a bowl and whisk to combine. Set aside.
In the bowl of your stand mixer (a handheld mixer works fine too!), cream the butter for on high for two minutes until it's light and fluffy. Add in sugar and continue to mix on high for another two minutes, scraping down the bowl and paddle as needed. Turn the mixer to low, add the egg and yolk and mix thoroughly. Scrape down the bowl and paddle and add the vanilla and sour cream. Mix for one minute on high, scraping down the bowl and paddle once more.
With the mixer on low speed, add in the dry ingredients and mix until just combined. Add the milk slowly and mix until just incorporated. Scrape down the sides and bottom of the bowl and give it a few stirs to make sure there are no lumps (without over-mixing). The batter will be slightly thick, but pourable.
Pour batter into prepared cake pans (about 2/3 of the way full) and bake for 25 minutes. They're done when they spring back to the touch and a toothpick inserted into the middle comes out clean. Let the cakes cool in the pan for five minutes before turning them out onto a wire rack for an additional few hours of cooling. Make sure they're entirely room temperature before filling and frosting with vanilla buttercream frosting.
Make Ahead Tips: these tiny cake layers can be made ahead and stored at room temperature, wrapped in plastic wrap, for up to two days. Alternatively, you can store the plastic wrapped layers in the freezer for up to two months. When you're ready to decorate, just thaw them at room temperature before filling and frosting.