1. Prepare the cupcakes. Preheat the oven to 350 degrees F and position a rack in the center. In a medium bowl, combine the flour, baking powder and salt; set aside. In the bowl of an electric mixer fitted with a paddle attachment, beat the butter, cream cheese and sugar on medium speed until light and creamy, 2-3 minutes. Add the eggs, one at a time, beating well after each addition. Add the limoncello and beat an additional minute. On low speed, add flour mixture and buttermilk alternatively, beginning and ending with the flour mixture. Add the lemon juice and zest and mix on low speed just until incorporated. Divide the batter between 12 muffin tins. Bake for 20 minutes or until a toothpick inserted near the middle comes out clean. Let cool completely.
2. Prepare the lemon curd. In a heavy medium saucepan, stir together the lemon zest, juice and sugar and bring to a simmer over medium heat. In a small bowl, whisk together the egg and egg yolk. Whisk the lemon mixture into the eggs to temper them. Scrape the mixture back into the saucepan and place over medium heat, whisking constantly until it thickens, about 5 minutes. Remove from the heat, and let cool completely.
3. Prepare the cream cheese limoncello frosting. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and cream cheese on medium speed until light and creamy, about 2-3 minutes. Add the limoncello and beat for an additional minute. Reduce the speed to low and gradually add the sifted powdered sugar and beat until fully incorporated and smooth.
4. Assemble the cupcakes. Cut a hole into each cupcake with the large end of a melon baller or the back end of a large pastry tip. Fill each cavity with the lemon curd. Fill a pastry bag with a star tip (I used Wilton 1M) with the cream cheese limoncello frosting and pipe onto each cupcake. Decorate with lemon and berries, if desired.