Combine flour, sugar, baking powder, and salt into the bowl of an electric mixer and mix to incorporate the dry ingredients.
With the mixer running on low speed, add the butter one tablespoon at a time, letting each mix about 20-30 seconds before adding the next. Mix until the butter and dry ingredients become a coarse crumb texture. Turn off mixer.
In a large measuring glass or bowl, beat together the 3 egg yolks and 1 whole egg.
Add the milk and vanilla.
Return the mixer to a slow speed and slowly add the wet ingredients to the dry. Turn the mixer to high and beat for 30 seconds until the batter is light and fluffy. There may still be small bits of butter, but that's okay!
Line a 12-cup cupcake pan with paper liners and fill each with batter. There should be enough to evenly fill each one. A large ice cream style scoop works extremely well for this.
Place in preheated oven and bake for 18-25 minutes. A wooden skewer inserted should come out clean with only a few crumbs.
Remove from oven and place cupcakes on a cooling rack. Cool completely before filling and frosting.
Once cooled, cut a circle (about 1 inch in diameter) in the top of each cupcake. Scoop out a small portion of the cake and fill with approximately one tablespoon of lemon curd.
Remove excess cake from the cut off piece and replace cake over the curd filling as a "cap."
Frost with the meringue frosting (recipe below).
For the meringue frosting
Combine the egg whites, sugar, corn syrup, salt, and cream of tartar in the bowl of an electric mixer (or any heatproof bowl).
Set over just-simmering water and stir constantly until the sugar dissolves and the mixture reaches 160°F on an instant-read thermometer.
Remove from heat and whip until stiff peaks are achieved and the mixture has cooled to room temperature, 4-6 minutes.
Add vanilla and mix until incorporated.
Place meringue frosting in a pastry bag fitted with a star tip. Pipe frosting onto prepared and filled cupcakes.
Using a kitchen torch (or an oven if extremely careful), lightly brown the meringue.