LEMON FILLING: In a small saucepan over low heat, whisk together egg yolks, sugar, flour, and salt. Gradually add lemon juice, lemon zest, water, and melted butter. Cook over medium heat, whisking constantly, until thickened (about 7 minutes). Remove from heat and set aside.

Preheat oven to 375°F.

CUPCAKES: In a medium bowl, beat butter and sugar until light and fluffy. Beat in eggs, lemon juice, baking powder, and salt. Gradually add flour and mix until just combined.

Divide cupcake batter among 12 cupcake liners in a muffin pan. Bake for 20 minutes or until cupcakes are firm. Remove from oven and cool for 5-8 minutes (do not turn off oven).

Prepare MERINGUE: Beat reserved egg whites with salt until stiff. Gradually add sugar and continue beating until stiff peaks form.

Using a sharp knife, remove the center of each cupcake (about 1" in diameter), leaving a layer of cupcake beneath the cutout. Fill cupcake centers with lemon filling.

Divide dollops of meringue between cupcakes, spreading out to edges and gently tapping with the back of a spoon to form peaks. Return cupcakes to the oven and bake for an additional 5-6 minutes until meringue peaks are golden brown.

Remove from oven and let cool for 10 minutes. Refrigerate for an hour or more before serving.