Preheat your oven to 350°F (175°C).

In a mixing bowl, combine the all-purpose flour and salt. Add the cold, cubed unsalted butter and mix with a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Gradually add the ice water, one tablespoon at a time, mixing until the dough starts to come together. Do not overwork the dough.

Turn the dough out onto a lightly floured surface and form it into a disk. Wrap it in plastic wrap and refrigerate for 30 minutes.

In a mixing bowl, whisk together the brown sugar, granulated sugar, eggs, vanilla extract, and salt until well combined.

Once the dough has chilled, roll it out to about 1/8-inch thickness on a floured surface. Using a round cutter, cut out circles of dough that are slightly larger than the muffin tin cups.

Fit the dough circles into the muffin tin, pressing down gently to cover the bottom and sides of each cup.

If using raisins or pecans, distribute them evenly among the pastry-lined cups.

Pour the filling mixture into each tart shell, filling them about ¾ full.

Bake in the preheated oven for 20-25 minutes, or until the filling is set and the edges of the pastry are golden brown.

Remove the butter tarts from the oven and let them cool in the muffin tin for about 10 minutes before transferring them to a wire rack to cool completely.