For the cake

1. Preheat the oven to 350ºF /180ºC.
2. Line 12 muffin or cupcake pans with paper liners.
3. In a large bowl beat butter while adding sugar gradually until creamy, 2-3 minutes.
4. Add zest and eggs, one at a time, beating well each time to combine.
5. Sift flour, baking powder, and salt (I do it directly over the butter mix) and add it in 3 parts to the butter mixture, alternating with the milk, lemon juice, and vanilla in 2 parts.
6. Mix well but don’t overbeat.
7. Divide cake batter evenly among the 12 paper cups.
8. Bake for 20-25 minutes, or until a tester comes out clean. 
9. Let cool completely on a wire rack.

For the lemon curd:

1. In a medium-heavy saucepan, heat lemon juice and butter just below the boiling point.
2. In a large bowl, mix eggs, yolks and sugar to blend.
3. Gradually add hot lemon mixture, mixing well.
4. Transfer the mixture back to the saucepan and stir constantly over medium heat until it thickens and leaves a path in the back of the spoon. Be careful not to burn the bottom.
5. Remove from heat, add salt and vanilla and mix.
6. Place a piece of plastic wrap or freezer sheets directly over the curd, covering it completely. This will prevent a layer from forming as it cools down. 
7. Refrigerate once it's cold. It keeps for 5 days in the refrigerator.

For the cream cheese frosting:

1. In a large bowl, mix butter, cream cheese, lemon juice, and zest. Beat until very smooth.
2. Add 1 cup sugar and beat well. Keep adding confectioners' sugar, ½ cup at a time, until the frosting is thick enough to pipe.
3. Transfer to a large piping bag with a star tip and refrigerate for 1 hour before using.

To assemble cupcakes:

1. With a small spoon or melon ball scoop, take out a piece of cake from the center of the cupcake. Discard.
2. Pipe or spoon some of the cold lemon curd, filling the hole completely.
3. Using a piping bag with a wide, star or plain tip, pipe the frosting on top of the cupcakes.
4. Sprinkle some lemon zest on top of the frosting before serving.