Preheat oven to 350°F. Grease two 8 or 9-inch round cake pans and set aside.
In a medium bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside.
Using a hand mixer or a stand mixer fitted with the paddle attachment, cream the butter with the sugar until smooth and lighter in color, about 2 minutes. Add the yogurt and vanilla extract and beat to incorporate.
Add the dry ingredients to the wet ingredients in thirds, alternating with the buttermilk (start and end with the dry mixture). Beat briefly between each addition to combine. Use a rubber spatula to fold in the sprinkles.
Divide the batter evenly between the two pans. Bake for 38-40 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 15 minutes before transferring to a wire cooling rack to cool completely.
To make the buttercream frosting:
Using a hand mixer or a stand mixer fitted with the paddle attachment, beat the butter until smooth and creamy, about 5-6 minutes.
Add the powdered sugar, vanilla, and salt and beat to combine. Add the milk and beat for another 5-6 minutes.
To assemble the cake:
Use a large serrated knife to level the top of each cake, if necessary, to create a completely flat surface. Place one layer on your cake stand and spread ¾ cup of frosting evenly on top. Position the second layer on top and evenly cover the top and sides with the remaining frosting.
The best sprinkles to use in funfetti cake batter are jimmies or quins. I recommend against using nonpareils, as they will likely bleed into the batter.
The cake must be COMPLETELY cool before assembling and frosting.
If you don't have buttermilk on hand, you can make your own. Add 1 tablespoon of white vinegar into a liquid measuring cup, then fill with whole milk until you have 1 cup. Allow the mixture to sit 5-10 minutes before using.
This cake can be made ahead of time. Once the cake layers have baked and cooled completely, cover tightly in plastic wrap and store at room temperature overnight. The frosting may also be made the night before and refrigerated in an airtight container, but I recommend making it fresh the day you are planning to assemble and serve the cake. If you do refrigerate the frosting overnight, allow it to come to room temperature before attempting to decorate the cake. You may need to beat it briefly to achieve that fluffy consistency again.