Lightly beat the 2 egg yolks in a small bowl and set aside. In a small saucepan over medium heat, stir together sugar and cornstarch. When heated, add in the water. Bring to a boil, stirring constantly until it thickens (1-2 minutes). Remove from the heat and spoon about 1/2 cup of the hot mixture into the small bowl with the egg yolks and stir quickly until combined. Return the egg mixture back to the pan and keep stirring. Bring the pan back to medium heat. Continue stirring and cooking until the curd thickens. Remove from heat. Stir in the butter, lemon juice, and lemon zest. Cover with plastic wrap and cool before using. Note: put the plastic ON the curd to prevent a skin from forming. When the cupcakes are ready, use a piping bag with a round tip to inject the cupcakes. 

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Lemon Cream Cheese Filling:

2 sticks of real (salted) butter, room temperature

4 ounces shortening

4 ounces cream cheese, room temperature 

1/2 tablespoon vanilla extract 

8-9 cups powdered sugar

1/2 cup fresh lemon juice

2-3 tablespoons milk 

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In a large mixing bowl, cream together butter, shortening, and cream cheese until smooth. Add in the vanilla and mix again. Begin adding powdered sugar, one cup at a time up to 8 cups. Your frosting will be very stiff at this point, so now add in the lemon juice and 2 tablespoons of milk. Mix and check the consistency. Add milk and powdered sugar as needed to get the right consistency that will hold its shape. Pipe onto cupcakes.