Prep: Preheat your oven to 350°F (175°C) and line your cupcake pan with liners.
Dry Ingredients: In a bowl, whisk together the cake flour, baking powder, and salt.
Creaming: In your mixer, cream the butter and sugar until pale and fluffy (about 3 minutes). Beat in the eggs one at a time. Add the lemon zest and juice.
Combine: Alternately add the dry ingredients and the sour cream to the butter mixture, starting and ending with the flour. Mix until just combined—don’t overmix!
Bake: Divide the batter into the cupcake liners (fill 2/3 full). Bake for 18–20 minutes, or until a toothpick comes out clean. Let cool completely before frosting.
Frosting: Whip the butter until light. Gradually add the confectioners' sugar, lemon juice, and heavy cream. Whip until smooth and creamy.
Decorate: Pipe the frosting onto the cooled cupcakes and garnish with a thin slice of lemon or extra zest.