Preheat oven to 350 degrees F. Lightly grease and flour (or line with paper or foil liners) a mini-muffin pan.
In a large mixing bowl, at a medium speed, cream the butter. Adding in a little sugar at a time, beat together until mixed and creamy. Add sifted cake flour, baking powder, and milk; beat until smooth. Add vanilla extract and set aside. In a separate dry bowl, beat the egg whites until soft peaks cling to the bowl (not stiff). Fold the egg whites into the butter/flour batter.
Divide batter among muffin cups, pour the batter almost to the top.
Bake approximately 20 to 25 minutes or until a toothpick wipes clean from the center. Remove from oven and let cupcakes cool before frosting them.
Prepare Basic Buttercream Icing. With a warmed flat knife, spread the icing over the top of each individual cupcake. Decorate with colored sprinkles or design of choice.
In a large mixing bowl, combine butter, milk, vanilla extract, and confectioner's sugar; mix at a low speed until smooth. If stiffer icing is needed or the weather is very warm, add a little extra sugar.
To create pastel colors: Divide prepared Buttercream Icing into a separate bowls for each desired color. Using a toothpick, add one drop of color, at a time, and mix until desired color is reached.
Yields 3 cups Buttercream Icing.
Mini Vanilla Cupcakes Recipe: https://whatscookingamerica.net/EllenEaston/VanillaCupcakes.htm