In a stand mixer, cream butter and sugar together. Add egg, sour cream and vanilla extract.
Whisk together dry ingredients (flour, baking soda, baking powder, lemon zest) in a separate medium bowl.
Alternate between adding dry ingredients to wet ingredients in a stand mixer and whisking in milk slowly.
Whisk until batter is smooth, but do not over-mix.
Pour batter into lined muffin tin, filling cupcake liners about halfway full.
Bake for 15-18 minutes until cupcake centers bounce back to the touch.
Allow cupcakes to cool on a baking rack.
Meanwhile, make the buttercream icing.
Lemon Buttercream Icing
Use a hand mixer to cream butter and vanilla.
Add ½ cup of powdered sugar.
Then add in lemon zest and juice.
Continue adding powdered sugar ½ cup at a time until the icing is thickened.
Add heavy cream. Icing should be smooth but hold its shape fairly well.
Pipe icing onto room-temperature cupcakes.
Notes
Make sure you bake cupcakes until they bounce back when gently touched or a toothpick comes out with cake crumbs on it. Batter on the toothpick means the cupcakes are under baked and a clean toothpick means the cupcake is too dry.For the best, fluffiest cupcakes, cooling at room temperature is recommended.