Beat softened butter with an electric mixer until creamy. Add powdered sugar and salt and mix on low until combined. Scrape down the side of the bowl as needed.

Mix in egg and vanilla until thoroughly combined.

Add the flour and mix on low until just combined, forming a soft, crumbly dough.

Turn out onto plastic wrap, press into a flattened disc, wrap, and chill for at least 1 hour.

Preheat oven to 350°F. Let the dough sit on the counter for 10 minutes before rolling. On a lightly floured surface, roll to ⅛ - ¼ inch thick and cut circles about 1 inch larger than your tart pans.

Press dough into the bottoms and sides of four 4-inch tart pans. Roll a rolling pin over the top edges to trim excess. Prick the bottoms with a fork. Line with foil and fill with pie weights.

Place the tarts on a cookie or baking sheet, then bake at 350°F for 15 minutes. Remove the foil and weights and bake for another 10 minutes, until the edges are golden brown. Cool for 5 to 10 minutes before removing from pans. Cool completely before filling. Wrap any remaining dough tightly and freeze for another use. Note: the pie weights will be hot, so use caution when removing them from the tart shells.

In a small saucepan, stir together the egg, yolks, sugar, lemon juice, and zest. Heat over medium heat, stirring constantly, until the mixture thickens to a custard-like consistency, about 7-8 minutes.

Remove from the heat and stir in the butter, one cube at a time, until fully melted and incorporated.

Strain through a fine mesh sieve into a bowl. Press plastic wrap directly onto the surface and cool to room temperature. Spoon filling into each tart shell until full. Chill for at least 4 hours until set.

Wipe down your saucepan, mixer bowl, whisk attachment, and thermometer with white vinegar. Combine sugar and water in the saucepan over medium heat. Do not stir the sugar-water mixture. In a stand mixer fitted with the whisk attachment, begin whipping egg whites on medium speed. When the syrup reaches 230°F, increase the mixer speed to high. When the syrup reaches 242°F, remove it from the heat and let the boiling slow slightly before pouring.

With the mixer running on high, slowly pour the syrup in a thin stream down the side of the bowl into the whipping whites. Immediately add the cream of tartar, lemon juice, and vanilla. Continue whipping until the bowl feels just slightly warm and the meringue is stiff, glossy, and holds firm peaks.

Pipe or spoon meringue over the chilled lemon filling. Toast with a kitchen torch until evenly golden. Serve immediately, or refrigerate, uncovered, for up to 2 hours before serving.