Mix sugar, flour, baking soda, salt, and butter in a medium-sized mixing bowl until fully combined.
Mix in egg.
Mix in lemon juice and milk.
Fill 6 clam shells with cupcake batter (about 3/4 of the way to the top of each shell). Place clam shells on a cookie sheet lined with parchment paper or a Silpat.
Put four cupcake liners in a cupcake tin and fill each 3/4 full with the remaining cupcake batter.
Bake shells and cupcake liners for 20 minutes or until cupcakes bounce back when lightly touched and cupcakes in the shells are just beginning to brown around the edges. The cupcakes in the liners may take a couple of minutes longer to bake than the ones in the shells.
Cupcake Crumb Topping Instructions
Remove the four baked cupcakes from the liners and convert them to cupcake croutons by following the directions in my cupcake crouton post. Skip the step about seasoning with brown sugar and spices because the end goal is dried-out cupcake crumbs.
Food process the croutons to make coarse crumbs.
Preheat oven to 375 F.
Place all cupcake crumb topping ingredients in a small saucepan on medium heat. Gently stir until butter is completely melted.
Divide topping evenly between all shells on top of the baked cupcakes.