In a large mixing bowl, combine the flour and salt. Add the chilled, cubed butter and cut it into the flour using your hands or a pastry cutter until the mixture resembles coarse crumbs with pea-sized pieces of butter. Gradually drizzle in the cold water, stirring with a spatula until the dough begins to clump. It may look loose—use your hands to gently gather it into a ball without overworking it.

Transfer the dough to a floured work surface and roll it into a 12x16-inch rectangle. Using a 4-inch cutter, such as an English muffin ring, cut out 12 circles. Collect and reuse any scraps by pressing them gently into the bases of the tart shells.

Lightly press each dough round into a standard muffin tin, forming it to fit the cups without stretching. Patch any thin spots at the base with dough scraps. Once all cups are filled, refrigerate the tin for at least 15 minutes. Meanwhile, preheat the oven to 425°F (220°C).

Toast the pecans in a dry skillet until fragrant, and if using raisins, soak them briefly in hot water, then drain and pat dry.

In a mixing jug or bowl with a spout, whisk together the brown sugar, salt, eggs, vinegar, and vanilla until well blended. Stir in the melted butter last to help bind the mixture. If using raisins and/or pecans, scatter them evenly into the bottoms of the chilled tart shells.

Carefully pour the filling over the fruit and nuts, filling each crust to about 1/2 to 3/4 inch from the top. The filling will rise and bubble as it bakes.

Bake the tarts in the preheated oven for 10 minutes at 425°F, then reduce the heat to 400°F (205°C) and bake for an additional 8 minutes. The tops should appear golden and slightly puffed.

Remove the pan from the oven and allow the tarts to cool in the muffin tin. If you're not using a non-stick pan, run a thin knife around each tart after a few minutes to prevent sticking, then let them cool completely in the tin before serving.