Preheat oven to 350°. Spray a 24-cup cupcake pan (regular-sized) with non-stick cooking spray, or grease with butter.

In a medium bowl, mix flour, baking powder, and salt.

In a large bowl, cream sugar, butter, and eggs. 

Next, add vanilla extract, lemon juice, and lemon zest.

On a low speed, alternate blending in dry mixture, and milk.

Add a few drops of yellow food coloring to give it a bright yellow color.

Fill each muffin cup with cupcake base until about 2/3 full and bake for 20-25 minutes until an inserted toothpick comes out clean. Cool thoroughly.

Once cupcakes are thoroughly cooled, hollow out a small well into the center of each and place one spoonful of lemon curd (enough to reach the top of the cupcake).

In a medium bowl, cream butter until it is soft and fluffy. 

Gradually blend in confectioners' sugar, then lemon juice.

Blend in milk one tablespoon at a time until right consistency is reached. 

Add a few drops of yellow food coloring to give the frosting a bright lemony yellow color.

Using a fat swirl, go about two swirls up on each cupcake with the frosting, working from the outside in.

Place a sugar cube and lemon slice into the top of each cupcake before serving.

Note: the lemon slice will last longer if you pat it dry before adding it. Otherwise, it might water down the frosting around it or slip off altogether.