Prepare the sugar cookie bases:
Preheat your oven to 350°F (175°C). In a large bowl, cream together softened butter and granulated sugar until light and fluffy. Add the egg, vanilla extract, and vegetable oil, mixing well. In a separate bowl, whisk together powdered sugar, flour, baking soda, and cream of tartar. Gradually combine dry ingredients with wet until a dough forms. Roll the dough into small balls and place on a parchment-lined baking sheet. Flatten slightly. Bake for 10–12 minutes or until edges are lightly golden. Let cool completely.

Make the lemon curd filling:
In a heatproof bowl, whisk together egg yolks, sugar, lemon juice, and zest. Place over a pot of simmering water (double boiler) and cook, stirring constantly until thickened (about 8–10 minutes). Remove from heat and whisk in cold butter until smooth. Let cool completely.

Prepare the meringue topping:
In a clean, dry bowl, beat egg whites until soft peaks form. Gradually add granulated sugar while continuing to beat until stiff, glossy peaks form.

Assemble the cookies:
Spoon or pipe a dollop of lemon curd onto each cooled cookie base. Top with a generous swirl of meringue.

Toast the meringue:
Use a kitchen torch to carefully toast the meringue until golden brown peaks form. Alternatively, place the cookies under a broiler for 30 seconds—watch closely to avoid burning.

Serve or chill:
Serve immediately or chill for an hour to let flavors meld. Best enjoyed within 1–2 days for maximum freshness.