Make the cupcakes: Preheat oven to 350°F. Place paper liners in the cavities of a standard size cupcake tin.
Combine the flour, baking powder and salt in a medium bowl. Whisk to combine.
In the bowl of an electric mixer, beat together the butter and sugar until fluffy. Add the egg and vanilla and mix to combine. Scrape down the edges of the bowl and mix again. In a glass measure with a pour spout, combine the milk and sour cream.
Beat in the flour mixture on low speed alternately with the milk mixture; begin and end with flour. Fill each cupcake liner 3/4 full of the batter. Bake for about 20 minutes, or until the cupcakes spring back when pressed in the centers. Remove from pan and let cool completely on a wire rack.
Make the frosting: In a stand mixer fitted with the whisk attachment, mix together the butter and confectioners’ sugar. Begin on low speed until crumbly, and then increase to high and beat for 3 minutes.
Add vanilla extract and beat again for another minute. If you find the buttercream is too stiff, you may add milk or heavy cream 1 tablespoon at a time until the mixture is spreading consistency.
Divide the frosting into separate bowls and tint as desired. Transfer frosting to a piping bag fitted with a large French decorator tip and pipe frosting onto the cupcakes. Immediately top with sprinkles if desired.
Keyword american buttercream, candy necklaces, yellow cake