01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed.
03 - In a large bowl, beat the room-temperature butter and granulated sugar together using a hand or stand mixer until the mixture becomes pale, light, and fluffy.
04 - Add the eggs one at a time, beating thoroughly after each addition to ensure full emulsification before adding the next.
05 - Mix in the lemon zest and fresh lemon juice until evenly combined throughout the batter.
06 - In a small bowl, stir together the milk and sour cream until smooth and well blended.
07 - Alternately add the dry flour mixture and the milk-sour cream mixture to the creamed butter, beginning and ending with the dry ingredients. Mix until just combined — do not overmix.
08 - Gently fold the diced fresh strawberries into the batter using a spatula, taking care not to crush the fruit or overwork the batter.
09 - Divide the batter evenly among the 12 prepared muffin cups, filling each liner about two-thirds full.
10 - Bake for 16 to 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 room-temperature butter on medium speed until smooth and creamy. Gradually add the sifted powdered sugar, beating until fully incorporated and fluffy. Mix in the strawberry puree, fresh lemon juice, and salt, then beat until the frosting is light and spreadable.
12 - Pipe or spread the buttercream onto the fully cooled cupcakes. Garnish with additional fresh strawberries or a sprinkle of lemon zest if desired.