Preheat oven to 350°F. Line cupcake tins with liners (makes 15-16 cupcakes).
Whisk flour, baking powder, and salt in a medium bowl.
Add melted butter to a large bowl and mix in sugar with a hand mixer for 30 seconds, until thick and yellow. Add eggs one at at time, beating after each addition. Beat in vanilla extract, lemon juice, and zest.
Add the flour mixture and milk alternately, starting and ending with the flour. Don’t over mix! Divide batter among liners, filling 2/3 full (about 1/4 cup of batter per cupcake). Bake for 13-18 minutes until a toothpick comes out clean. (Mine took about 14 minutes.) Cool completely before frosting.
To fill: cut out a small bit of the top/center of the cupcake. (Cut a hole that’s about the size of a dime and the amount of cupcake you take out is a little larger than a marble.) Fill each hole with about 1/2 - 1 tablespoon of lemon curd. Replace a piece of the cupcake to cover.
To make the frosting: Beat butter until creamy. Beat in salt and 1 cup of powdered sugar at a time until it’s all added.
Beat in lemon juice and zest. Add 1 tablespoon of heavy whipping cream and mix until smooth. Add more cream as needed for desired consistency. Frosting can be made up to 2 days ahead and stored in the refrigerator. Make sure to let it come to room temperature before using.
Frost cupcakes by placing frosting in a pastry bag. I like the 1M tip, it gives pretty swirls. Add extra lemon zest or sprinkles for decoration, if desired.