Preheat oven to 350°F and prepare 6, 4-inch tartlet pans with cooking spray. Set aside.In a large mixing bowl with a hand mixer, beat the butter, sugar, and vanilla extract until completely combined. Add the flour and fold in with a spoon until completely combined and a crumbly dough forms.Firmly press the dough evenly over the bottom and along the sides of the pans. Trim off any excess.Bake for 10-15 minutes or until the crust begins to turn golden. Cool for at least 15 minutes before removing the shells from the pans.
Making the lemon curd
Over a double broiler on medium heat, whisk together the sugar, lemon zest, eggs, and lemon juice. Whisk constantly until the mixture becomes thick (could be about 10-15 minutes). The mixture will thicken more once cool.Remove from heat and immediately start adding the butter, cut into cubes a few cubes at a time, and whisk until the butter is melted and completely incorporated. Allow the filing to cool to room temperature before filling the tart shells.Fill the tart shells with the lemon filling, then refrigerate for at least 2 hours before serving.