Preparation:

Step 1: Prepare the Lemon Curd

1. In a small saucepan over medium heat, whisk together the lemon juice, zest, sugar, and egg yolks.
2. Cook, stirring constantly, until the mixture thickens, about 5-7 minutes. It should coat the back of a spoon.
3. Remove from heat and stir in the butter until smooth.
4. Strain the curd through a fine-mesh sieve to remove any lumps.
5. Let the curd cool, then refrigerate for at least 1 hour until thickened.

Step 2: Make the Lemon Cupcakes

1. Preheat your oven to 350°F (175°C) and line a cupcake pan with paper liners.
2. In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
3. In a large bowl, cream together the butter and sugar until light and fluffy.
4. Beat in the eggs, one at a time, then add the vanilla extract.
5. Mix in the lemon juice and zest.
6. Gradually add the flour mixture, alternating with milk, beginning and ending with the flour mixture. Mix until just combined.
7. Fill the cupcake liners about 2/3 full with batter.
8. Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
9. Let cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

Step 3: Fill the Cupcakes with Lemon Curd

1. Once cupcakes are fully cooled, use a small knife or cupcake corer to remove the center of each cupcake.
2. Fill each hole with a spoonful of chilled lemon curd.
3. Replace a small portion of the cupcake tops to cover the filling.

Step 4: Prepare the Frosting

1. In a large bowl, beat butter until smooth and creamy.
2. Gradually add powdered sugar, one cup at a time, beating well after each addition.
3. Add lemon juice, vanilla extract, and heavy cream (or milk) and beat until smooth and fluffy.
4. Adjust consistency by adding more powdered sugar if too thin, or more cream if too thick.

Step 5: Frost and Decorate

1. Transfer the frosting to a piping bag fitted with a decorative tip.
2. Pipe the frosting onto each cupcake in a swirl pattern.
3. Garnish with lemon zest, lemon slices, or a drizzle of lemon curd.