Make the dough: Add flour, sugar, and salt to a food processor. Give the dry ingredients a whirr to blend them.
Add the butter. Pulse for 5-6 one second pulses, or until the butter is cut it with no large chunks remaining.
Add the egg yolk, heavy cream, and vanilla. Blend for 30 seconds.
The dough should begin to ball up and come together. If it still looks dry, add one more tablespoon of cream and pulse a few times to mix it in.
Chill the dough: Once the dough has begun to come together, transfer the dough to a bowl and use your hands to shape it into one disc for a 9-inch tart or 6 discs for mini tarts.
Cover the disc with plastic wrap and let chill for 1-2 hours, or up to 3 days. If chilling more than 1 hour, let the dough warm up a little and become pliable before rolling.
Roll the dough: Before rolling out the dough, butter each of the tart shells. If needed, cut out parchment rounds for the bottom. Dust the dough with flour and roll out the dough to fit the tart shells. Once the dough is in the shell, use the rolling pin over the top the tart pan to neatly cut the edges.
Press holes into the tart dough all over the surface with a fork. Freeze the dough in the tart pans for 30 minutes.
Preheat the oven to 350F.
Bake the crust: If making mini tarts you can put them directly into the oven as is. If making a 9-inch tart, line the tart dough with foil and fill with pie weights.
For mini tarts, bake for 15 minutes. For a 9-inch tart, bake for 10 minutes with the pie weights and then remove them and bake for 7-8 minutes more. Let cool to room temperature. When ready to fill, remove the tart shells from the pans.