In the bowl of stand mixer, cream together the butter, baking powder, sugar, salt and vanilla extract on medium until fluffy, about 5 minutes. Scrape sides as needed.

Decrease speed to low and add the egg whites, one at a time, beating well with each addition.

Add in ⅓ of the flour, then half the milk, another ⅓ of flour, the remaining milk, and then the remaining flour. Scrape sides as needed.

Fill the cupcake liners with ¼ cup of batter.

Bake for 18-22 minutes or until the cupcakes start to brown and a toothpick poked through the center of a cupcake comes out clean. Remove from oven, cool for 5 minutes in the pan. Then remove from the pan and cool on a wire rack. Frost when the cupcakes have completely cooled.

Beat the butter at medium speed with an electric mixer until creamy (1-2 minutes.) Gradually add the powdered sugar, alternating with lemon juice, beating at low speed until blended.

Cut both ends off of each lemon. Use a sharp knife to remove just the peel, leaving as much of the white pith on the lemon as possible.

Bring 2 cups water and the lemon peel to a boil in a small pot. Drain the water and repeat with fresh cold water 2 more times. Drain the water and set the peels aside.

Combine the remaining 2 cups of water and the sugar in the pot. Bring to a boil and stir to dissolve the sugar. Reduce heat to low and stir in the lemon peels. Simmer for 5-10 minutes or until the peels are translucent and soft. Remove the peels and allow to completely dry. Toss the dried peels in the extra sugar.

To prepare your cupcakes, use a sharp knife to carve out the centers of each cupcake. Fill each cavity with lemon curd, then frost the cupcakes. Top the frosting with candied lemon peel.