01 - Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper cupcake 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 the mixture becomes pale, light, and fluffy—approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating well after each addition to ensure full emulsification before adding the next.
05 - Mix in the lemon zest and fresh lemon juice until evenly incorporated throughout the batter.
06 - In a small bowl, stir together the milk and sour cream until smooth and combined.
07 - Alternately add the dry flour mixture and the milk-sour cream mixture to the creamed butter base, beginning and ending with the dry ingredients. Mix on low speed after each addition until just combined—do not overmix.
08 - Using a spatula, gently fold the diced fresh strawberries into the batter with careful, sweeping motions to avoid crushing the fruit or deflating the batter.
09 - Divide the batter evenly among the 12 cupcake liners, filling each approximately two-thirds full to allow room for rising.
10 - Bake on the center oven rack 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 before frosting.
11 - Beat the room-temperature butter on medium speed until smooth and creamy. Gradually add the sifted powdered sugar, beating on low speed until incorporated, then increase to medium-high and beat until fluffy. Add the strawberry puree, fresh lemon juice, and a pinch of salt. Beat on medium-high speed for 2 minutes until the frosting is light, creamy, and pipeable.
12 - Pipe or spread the buttercream generously onto the fully cooled cupcakes. Garnish with additional fresh strawberry slices or a sprinkle of lemon zest if desired. Serve at room temperature.