In a food processor, combine all-purpose flour, sugar, and fine sea salt. Add cold butter chunks and pulse until resembling coarse crumbs. Gradually drizzle in cold water while pulsing until a dough forms. Wrap and refrigerate for at least 1 hour.

Sprinkle flour on a clean surface and roll out the chilled dough to 1/8 inch thick. Cut out 15 circles and press them into a greased muffin tin. Refrigerate the crusts.

Preheat the oven to 350°F (175°C) to ensure even baking.

Cream room-temperature unsalted butter and brown sugar until smooth. Add eggs one at a time, mixing well. Stir in maple syrup, salt, and vanilla extract. Let sit for 5-10 minutes.

Incorporate optional ingredients like raisins or nuts into each tart crust before pouring in the filling.

Pour filling into each tart shell, about three-quarters full. Bake for 25 minutes until edges are caramelized and filling is set.

Allow tarts to cool in the pan for 5-10 minutes before transferring to a wire rack to cool completely.