In a mixing bowl, use an electric mixer to beat softened butter until creamy. Add in the powdered sugar and salt and mix on low until combined. 

Mix in the egg and vanilla extract until thoroughly combined. Finally, add in the flour and mix on low until just combined into a dough that is soft and crumbly.

Turn the dough out onto a sheet of plastic wrap. Use your hands to press the dough together into a flattened disc. Wrap in plastic wrap and chill in the fridge for at least 1 hour or overnight.

Preheat oven to 350°F. Let the chilled dough sit on the counter for 10 minutes before rolling.

Rolling dough about ⅛" thick. Use a knife to cut out a rough circle about 1" bigger than your tart pans. Re-roll the scraps for the rest of your tarts.

Place the sheet of dough into the tart pan and press the dough into the bottom and side ridges of the pan. Use your rolling pin to roll over the top edges of the pan to "cut" the excess dough off. 

Use a fork to prick some holes in the bottom of the tart shell.

Gently form a piece of aluminum foil into the tart shell and fill with pie weights or uncooked rice. Repeat with all other mini tarts.

Bake for 15 minutes, then remove the foil and weights and bake for another 10 minutes until the edges are golden brown. Let the tart shells cool for 5- 10 minutes before removing them from the tart pans to cool completely.

In a small pot, stir together all the eggs, sugar, and lemon juice.

Heat on medium heat, stirring constantly until the mixture thickens considerably into a custard consistency, about 15 minutes.

Stir in the butter, one cube at a time, until all the butter has melted and incorporated. 

Transfer filling to a bowl and press a piece of plastic wrap onto the surface of the custard to prevent a skin from forming while it's cooling.

Once the lemon filling has cooled to about room temperature, spoon the filling into each tart shell until full. Chill the tarts in the fridge for at least 4 hours or overnight until set.

In a pot, bring about 1" of water to a simmer.

In a large mixing bowl, stir together the egg whites and sugar. Place the bowl on top of the pot of simmering water to create a double boiler. Make sure the bottom of the bowl does not touch the water.

Heat the egg and sugar mixture while stirring constantly until all the sugar has dissolved, about 4 minutes. When you rub a small amount of the syrup between your fingers, it should be smooth without any grittiness.

Remove the bowl from the pot. Add in the vanilla extract. Use an electric mixer to whisk the egg whites on high speed until it becomes stiff peaks, about 5-7 minutes.

Transfer the meringue to a piping bag fitted with a piping tip of your choice. Pipe the meringue on top of the lemon filling. Use a kitchen torch to toast the meringue before serving.