lemon curd filling:
1/2 cup lemon juice
zest of two lemons
1/4 cup sugar
1 egg
1 egg yolk
Mix lemon juice, lemon zest, and sugar of medium heat until boiling. Whisk egg and egg yolk. Whisk lemon mixture into eggs off the heat (to temper). Add all back to sauce pan and cook over medium heat, whisking constantly, until thickened. Cool completely.
Use a melon baller to scoop a hole into tops of cooled cupcakes. Fill with cooled lemon curd.
lemon buttercream icing:
1/2 stick butter
6 oz cream cheese
4 cups powdered sugar
2 tablespoons limoncello
Beat butter and cream cheese. Add limoncello and beat until smooth. Beat in powdered sugar until desired icing consistency is obtained. Ice filled cupcakes. Garnish with lemon slices.