01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat the butter and granulated sugar together using a hand or stand mixer until pale, light, and fluffy.
04 - Add the eggs one at a time, beating well after each addition until fully incorporated.
05 - Mix in the lemon zest and fresh lemon juice until evenly distributed.
06 - Combine the milk and sour cream in a small bowl and stir until smooth.
07 - Alternately add the dry ingredient mixture and the milk mixture to the creamed butter, beginning and ending with the dry ingredients. Mix until just combined—do not overmix.
08 - Gently fold the diced strawberries into the batter using a spatula.
09 - Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
10 - Bake for 16–18 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Transfer to a wire rack and cool completely.
11 - Beat the butter on medium speed until smooth and creamy. Gradually add the sifted powdered sugar, beating until fully incorporated and fluffy. Add the strawberry purée, lemon juice, and salt, then beat until light and creamy.
12 - Pipe or spread the buttercream onto the fully cooled cupcakes. Garnish with additional fresh strawberries or lemon zest if desired.