01 - Preheat the oven to 350°F. Line a standard 12-cup muffin tin with paper liners.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed. Set aside.
03 - In a large mixing bowl, beat the room-temperature butter and granulated sugar together using a hand or stand mixer on medium-high speed until pale, light, and fluffy—about 3 minutes.
04 - Add the eggs one at a time, beating well after each addition until fully incorporated. Mix in the lemon zest and fresh lemon juice.
05 - In a small bowl, stir together the milk and sour cream until smooth.
06 - With the mixer on low speed, alternate adding the dry ingredient mixture and the milk-sour cream blend to the creamed butter, beginning and ending with the dry ingredients. Mix until just combined—do not overmix.
07 - Using a spatula, gently fold the diced strawberries into the batter with minimal strokes to avoid crushing the fruit and deflating the batter.
08 - Divide the batter evenly among the prepared muffin cups, filling each liner about two-thirds full.
09 - Bake for 16–18 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Transfer the tin to a wire rack and let the cupcakes cool completely before frosting.
10 - In a mixing bowl, beat the room-temperature butter on medium speed until smooth and creamy. Gradually add the sifted powdered sugar, beating on low until incorporated, then increasing to medium-high until light and fluffy. Add the strawberry purée, fresh lemon juice, and a pinch of salt; beat until the frosting is silky and spreadable.
11 - Pipe or spread the buttercream generously onto each cooled cupcake. Garnish with fresh strawberry halves, additional lemon zest, or a thin slice of candied lemon if desired.