Preheat the oven to 375°F (190°C).

In a mixing bowl, combine flour and cold butter; mix until crumbly.

Gradually add cold water until the dough holds together. Shape into a disk and refrigerate for 30 minutes.

Roll out the chilled dough to about 1/8 inch thick and cut out circles.

Press circles into muffin pans to form tart shells.

In another bowl, whisk together brown sugar, granulated sugar, eggs, vanilla extract, and salt until smooth.

Pour filling into each tart shell and bake for 20-25 minutes until set but slightly wobbly.

Allow to cool before serving warm.