Make the pastry first. Cube the butter and place in the freezer to get extra cold. Place flour, sugar, and salt in a stand mixer fitted with a paddle. Add cold cubed butter and turn mixer on low to mix the butter chunks into the flour. Add in ice water, 1 Tbsp. at a time until mixture appears shaggy. Avoid the urge to add more water. Dump mixture onto a clean surface. Wet your hands and form mixture into a large flat disk. Wrap disk in plastic and chill in the refrigerator for 20 minutes.
Spray a muffin tin with non-stick baking spray. Un wrap dough and roll out onto a clean work surface. Roll dough to roughly 10”x10”square. Fold the left side into the middle of the dough, then fold the right side over the left side. Rotate dough 45 degrees, then repeat folds 2 more times. Roll dough out one last time, to about 1/8” thickness. Use a 4” cookie cutter to cut out 12 circles, you may need to re roll the dough to get 12. Fit each circle of dough into each muffin tin cavity. Freeze the tart shells while preparing the filling.
Preheat oven to 425 degrees Fahrenheit. In a mixing bowl, combine melted butter, brown sugar, corn syrup, salt, and vanilla. Whisk together. Add egg then whisk together. Fill each tart shell base with raisins. Pour the filling over top, filling up about ½way on each tart. Bake tarts for 14-15 minutes, or until the edges of the shell appear golden brown. Remove pan from the oven. Run a knife around the tart shells. Cool tarts at room temperature in the muffin tin completely before trying to remove, about 2 hours. Use an offset spatula to lift tarts out of the muffin tin. Serve on a plate, enjoy!