Make the Lemon Cupcakes

1. Preheat the oven to 350°F (176°C) and prepare a cupcake pan with liners.
2. Add the flour, sugar, baking powder and salt to a large mixer bowl and combine. Set aside.
3. Add the milk, vegetable oil, vanilla extract, lemon zest and egg to a medium sized bowl and combine.
4. Add the wet ingredients to the dry ingredients and beat until well combined.
5. Slowly add the water to the batter and mix on low speed until well combined. Scrape down the sides of the bowl as needed to make sure everything is well combined.
6. Fill the cupcake liners about half way and bake for 15-17 minutes, or until a toothpick comes out with a few moist crumbs.
7. Remove the cupcakes from the oven and allow to cool in the pan for 2 minutes, then remove to a cooling rack to finish cooling.

Make the Lemon Curd

1. Place a saucepan of water over medium heat and bring it to a simmer.
2. Place a metal mixing bowl (or the top of a double broiler over the simmering water and combine all the lemon curd ingredients. Heat while whisking constantly until mixture thickens and reaches 160°F (71°C), or coats the back of a spoon.
3. Pour the lemon curd into a small bowl and cover with clear wrap pressed against the top of the curd to prevent a film from forming. Refrigerate until completely cool and firm, 3-4 hours.

Make the Meringue Frosting

1. When you are ready to build the cupcakes, make the meringue frosting. Place a saucepan of water over medium heat and bring it to a simmer.
2. Combine the sugar, cream of tartar, vanilla extract and egg whites in a large metal mixer bowl (or the top of a double broiler). Place it over the saucepan of simmering water.
3. Whisk constantly until sugar is dissolved and egg whites are warm, to 160°F (71°C) on a thermometer, about 4-6 minutes. Remove from heat.
4. Using the whisk attachment, beat the egg white mixture on low speed, gradually increasing to high, until stiff, glossy peaks form, 5 to 7 minutes.

Assemble the Cupcakes

1. Fill a piping bag fitted with a large round tip (I used Ateco tip 808) with meringue frosting. Set aside.
2. Use a cupcake corer (or a knife) to cut a hole in the center of the cupcakes.
3. Fill the center of the cupcakes with lemon curd.
4. Pipe a swirl of meringue frosting on top of each cupcake. 
5. If desired, cut thin slices of lemon to finish off the cupcakes. Insert the lemon slices gently into the frosting, then toast the frosting lightly with a kitchen torch.
6. Refrigerate cupcakes until ready to serve. Best if eaten within 2-3 days.