Step 2

Make the Crust: In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy, about 3 minutes. Add the egg and vanilla; mix to combine. Scrape the bowl well.

Step 3

Add the flour, baking powder and salt; mix to combine. Divide the dough into 12 even pieces.

Step 4

Press each piece evenly into the cavities of a 12-cup muffin pan, taking care to press into the corners and maintain the same thickness all over. Chill for 15 minutes. Preheat the oven to 375°F.

Step 5

Make the Filling: In a medium bowl, whisk the lemon juice, sugar, eggs and egg yolks until well combined. Add the mascarpone and salt; whisk until combined.

Step 6

Pour the custard into a vessel with a pour spout (such as a liquid measuring cup). Gently pour the custard into each chilled crust, filling just under the inner edge of the crust.

Step 7

Bake until the crust is golden and the custard is set, 30 to 35 minutes. Run an offset spatula around the outer edge of each tart and then let cool completely.

Step 8

Make the Meringue: In the bowl of an electric mixer fitted with the whip attachment, place the egg whites and cream of tartar.

Step 9

In a small pot, combine the sugar with 3 tablespoons water. Stir the mixture over medium heat until it begins to come to a simmer, then stop stirring. Continue to cook until the sugar dissolves and the mixture reads 140°F on a thermometer.

Step 10

When the mixture reaches 140°F, remove it from the heat and begin to whip the egg whites on medium-high speed. When the egg whites are frothy and starting to turn white, begin to add the sugar mixture in a slow, steady stream. Continue to whip until the mixture reaches medium peaks.

Step 11

Apply the meringue to the surface of the tarts with a spoon, an offset spatula or a piping bag as desired. Use a kitchen torch to toast the meringue.