Whisk the flour and salt together. Cut in the cold cubed butter until the mixture has pea-sized pieces, then add ice water a tablespoon at a time until the dough just holds together. Shape it into a disk, wrap, and chill for about 30 minutes.

Lightly grease a standard 12-cup muffin tin. Roll the chilled dough to approximately 1/8 inch (3 mm) thick and cut twelve rounds large enough to line the muffin cups.

Press each pastry round gently into a muffin cup without stretching the dough. Whisk together the melted, slightly cooled butter, brown sugar, corn syrup, egg, vanilla, and salt.

Place a few raisins, currants, or pecans in each pastry shell if using. Pour the filling into the shells, filling each about two-thirds full because the mixture bubbles as it bakes.

Bake at 375°F (190°C) for 18–22 minutes, until the pastry is golden and the filling is puffed and set around the edges but still slightly wobbly in the center. Cool in the pan for at least 10–15 minutes before removing.