Preheat oven to 350°F. Line a cupcake pan with 8 paper liners.

In a small bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl or stand mixer, cream butter and sugar until light and fluffy. Mix in the egg, lemon zest, lemon juice, and vanilla until smooth, scraping the bowl as needed. Alternate adding spoonfuls of the flour mixture and sour cream, mixing until batter is smooth with no streaks.

Divide batter evenly between the 8 liners, filling each about two-thirds full. Bake for 18 to 20 minutes, or until a toothpick comes out with just a few moist crumbs. Cool completely on a wire rack.

Using a sharp knife or cupcake corer, remove the center of each cooled cupcake, leaving the bottom intact. Fill each cavity with lemon curd. Refrigerate until ready to frost.

In a heatproof bowl set over simmering water, whisk together the egg whites, sugar, and cream of tartar until warm to the touch and the sugar is fully dissolved. Remove from heat and beat with a hand mixer or stand mixer on high speed until glossy and stiff peaks form. Add vanilla during the last 30 seconds.

Transfer meringue to a piping bag. Pipe onto chilled cupcakes and lightly char with a kitchen torch. Serve immediately.