Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners and set aside. In a medium bowl, whisk together the flour, baking powder, and salt until well combined. Set this mixture aside. In a large measuring cup or bowl, mix the sour cream, milk, and egg whites until fully blended; set aside. In a large bowl or the bowl of a stand mixer fitted with a paddle attachment, beat the melted butter and sugar on medium speed for about 2 minutes until well combined. Add the lemon juice and lemon zest, then continue beating for another minute on medium speed. Gradually add the flour mixture to the butter and sugar mixture, beating just until combined. Then add the sour cream, milk, and egg mixture and beat on low speed until just combined—be careful not to overmix, as this can make the cupcakes dry and dense.

Divide the batter evenly among the 12 prepared muffin cups and bake for 18–20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow the cupcakes to cool completely before frosting.

To prepare the Lemon Buttercream, beat the butter until light and fluffy. Gradually add 4 cups of confectioners’ sugar, mixing on low speed until well incorporated. Add the lemon juice, lemon zest, and heavy cream, then beat until the frosting is smooth and combined. Add more confectioners’ sugar if needed until the frosting reaches your desired consistency. Transfer the frosting to a piping bag or spread it generously over the cooled cupcakes.