Combine the egg yolks and sugar in a medium mixing bowl and whisk vigorously until well combined and smooth. Whisk in the lemon juice and zest. Pour the mixture into the top of a double boiler set over just simmering water. Stir until mixture is light yellow and thickens enough to coat the back of a wooden spoon—about 15 to 20 minutes. Remove from the heat and whisk in the butter 1 tablespoon at a time, stirring to melt each pat of butter before adding the next.

Scrape into a clean mixing bowl and cover with plastic wrap, pressing the wrap onto the surface of the lemon curd to prevent a skin from forming. Refrigerate until completely chilled, or up to 2 weeks.

Preheat the oven to 350 degrees. Line 16 muffin cups with cupcake liners and set pans aside.

Whisk the flour, baking powder, baking soda, and salt together in a medium mixing bowl.

Place the butter in a large mixing bowl and beat with an electric mixer set to medium speed until very creamy. Add the sugar and vanilla and beat until well combined. Scrape down the sides and bottom of the bowl, then add the eggs 1 at a time, beating well after each addition. Add the flour mixture in 3 additions, alternating with the sour cream, beginning and ending with the flour. (It will go flour, sour cream, flour, sour cream, flour.) Fold in the lemon zest; batter will be very thick.

Fill the prepared muffin cups with about 1/4 cup of batter and bake 20 to 25 minutes, until a toothpick inserted into the center of the cupcakes comes out clean. Cool completely on wire racks before filling and frosting.

Beat the butter in a mixing bowl with an electric mixer until very creamy. Add 3 cups of the powdered sugar, 2 tablespoons of milk, and the salt. Mix on low until combined. Scrape down the sides and bottom of the bowl, then beat in the lemon curd. Turn mixer to medium and beat until smooth. Add more milk if frosting is too thick and more powdered sugar if frosting is too thin.

Assembly:

Use a small paring knife to cut a hole in the center of each cupcake, about 1 inch across and 1 inch deep—reserve the cake you pull out of the holes. Spoon the remaining lemon curd into a pastry bag or large zip-top plastic bag and pipe the curd into the cupcakes. Slice the bottom off the the cake “plugs” you cut out of the cupcakes, and place one on top of the curd in each cupcake.

Frost the filled cupcakes with the lemon buttercream and serve.