Preheat convection oven to 350°F (175°C). Line two heavy-gauge baking sheets with silicone mats or parchment paper.

In a stand mixer fitted with the paddle attachment, cream 1 cup butter and 1 cup sugar on medium-high speed for 3 minutes until aerated. Incorporate the egg and vanilla.

Sift together flour, baking powder, and salt. Reduce mixer speed to low and gradually add dry ingredients until just combined.

Portion dough into 30g spheres. Place on baking sheets and use a dampened thumb to create a deep well in the center of each. Bake for 10-12 minutes until edges are set but not browned. Cool completely on wire racks.

To prepare the curd, whisk egg yolks, 0.5 cup sugar, lemon juice, and zest in a heavy-bottomed saucepan over medium-low heat. Stir constantly with a silicone spatula until the mixture reaches 170°F (77°C).

Remove curd from heat and emulsify the cubed butter into the mixture one piece at a time. Strain through a fine-mesh sieve into a bowl, cover with plastic wrap touching the surface, and refrigerate until set.

For the meringue, combine egg whites, 0.75 cup sugar, and cream of tartar in a heat-proof bowl. Place over a bain-marie and whisk until the temperature reaches 160°F (71°C) and sugar crystals have dissolved.

Transfer the egg white mixture to a stand mixer with a whisk attachment. Whip on high speed until stiff, glossy peaks form and the bowl feels cool to the touch.

Deposit 1 teaspoon of chilled lemon curd into the center of each cooled cookie.

Transfer the meringue to a piping bag fitted with a star tip and pipe over the curd. Use a culinary torch to apply direct flame to the meringue until evenly toasted and golden brown.