In a food processor, combine all-purpose flour, sugar, and fine sea salt. Add cold unsalted butter and pulse until it resembles coarse crumbs. Gradually drizzle in cold water until the dough comes together. Shape into a disc, wrap in plastic, and refrigerate for 1 hour.

On a floured surface, roll out the chilled dough to 1/8-inch thickness. Use a round cutter to make 15 rounds and press them into a greased muffin pan. Refrigerate again while preparing filling.

Cream together softened unsalted butter and brown sugar until smooth. Add eggs one at a time, mixing thoroughly. Pour in maple syrup, fine sea salt, and vanilla extract, stirring until well combined. Let sit for a few minutes.

Add optional raisins or nuts to each crust. Carefully pour the filling into each tart shell, filling three-quarters full. Bake in a preheated oven at 350°F for 25 minutes or until filling is set.

Remove tarts from oven and cool in the pan for 10 minutes. Transfer to a wire rack to cool completely. Serve warm or at room temperature.