Preheat oven to 350 degrees Fahrenheit.

Make the batter. Whisk together flour, baking powder, baking soda, and salt. Set aside.

Cream butter and sugar using a hand mixer until light and fluffy, about 5 minutes.

Beat in eggs and additional yolk with the vanilla extract.

Add the dry mixture in alternatively with buttermilk and continue to beat, about 1 minute.

Divide batter into prepared mini cake pan wells. Smooth the top and bake for 20-25 minutes.

While mini cakes bake, make the buttercream frosting. Cream butter in a mixing bowl fitted with the paddle attachment. Gradually beat in powdered sugar until fully incorporated. Beat in vanilla extract and salt.

Add milk a little at a time until the frosting reaches preferred consistency. Set aside until ready to use.

Remove cakes from the oven and place on a wire rack for 5 minutes before turning out. Place on a wire rack and cool completely before frosting to your heart’s desire.