Preheat Oven: Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with cupcake liners.
Infuse Sugar with Lemon Zest: In a food processor, pulse together granulated sugar and lemon zest until the sugar turns light yellow and the zest is finely incorporated.
Mix Wet Ingredients: In a medium bowl, whisk together melted butter, eggs, milk, sour cream, lemon juice, and the lemon-infused sugar.
Sift Dry Ingredients: In a large bowl, sift together all-purpose flour, baking powder, baking soda, and kosher salt. Mix to combine.
Combine Wet and Dry Ingredients: Gradually add the wet mixture to the dry ingredients. Stir gently until just combined. Be careful not to overmix.
Fill the Cupcake Liners: Fill each cupcake liner about 3/4 full with the batter. Bake for 5 minutes at 375°F, then reduce the temperature to 350°F (175°C) and bake for an additional 10-12 minutes. Cupcakes are done when a toothpick inserted into the center comes out clean.
Cool the Cupcakes: Let the cupcakes cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.
Make the Frosting: Beat softened butter and lemon zest in a stand mixer for 2-3 minutes. Gradually add powdered sugar in batches, mixing until smooth. Add lemon juice and mix until fully combined.
Decorate: Once the cupcakes have cooled, pipe the buttercream frosting on top using a piping bag fitted with your favorite tip. Garnish with extra lemon zest or lemon slices if desired.