Lay the first pie crust out on a lightly floured work surface (leave the second crust in the refrigerator until you are ready for it). Roll it into a 12-inch circle, working from the center out. Grab 2 standard 12-cup muffin pans (no need to grease).
Flour a 4-inch round cookie cutter or biscuit cutter (if you don’t have one, use a drinking glass). Stamp out 7 rounds from the pie crust (reroll the scraps if needed). Gently place each round into a well of the muffin pan and press the dough up the sides to create a tart shell.
Repeat with the second pie crust, filling the remaining wells. Place the pans in refrigerator and chill for at least 30 minutes or cover and chill overnight. (If you only have one muffin pan, you can bake the tarts in batches. Be sure to let the pan cool all the way in between).
When ready to bake, position racks in the upper and lower thirds of the oven and preheat the oven to 350 degrees F. Place the raisins in a small, microwave-safe bowl and stir together with the bourbon. Microwave on high for 30 seconds. Set aside.
Cut the butter into a few pieces and place in a large microwave-safe mixing bowl. Microwave on medium heat, just until melted. Let cool to room temperature. To the bowl with the butter, add the brown sugar and flour.
Whisk to combine until smoothly blended (it will be thick).
Whisk in the eggs, maple syrup, vanilla, and salt until evenly combined.
Scatter the raisins evenly over the tart shells.
Divide the filling between the crusts, filling each well 3/4 of the way to the top (use a small ladle or transfer the filling to a measuring cup with a spout, then pour it into the crusts).
Bake for 25 to 35 minutes, or until the top is dry, crackly, and set. The filling should jiggle only a little in the center when you wiggle the pan, and the crust is turning golden. The easiest way to tell when the tarts are done is with an instant-read thermometer. The filling should register 200 degrees F.
Place the pan on a wire rack and let the tarts cool completely before unmolding them. The filling will settle and crackle. To unmold, use a dull knife such as a butter knife to loosen the tarts from the muffin wells as needed. Gently lift the tarts onto the rack.
Notes
*If using nuts instead of raisins, skip microwaving with the bourbon. Reduce the maple syrup by 1 tablespoon and add the bourbon with the maple syrup. You also can use nuts (or no mix-ins) and omit the bourbon entirely.
TO MAKE AHEAD: Line the muffin tin with the crusts and refrigerate for up to 1 day. Combine the filling (except for nuts if using) up to 1 day ahead, cover tightly, and refrigerate. Fill the crusts just before baking.
TO STORE: Keep leftover butter tarts at room temperature for up to 3 days. Enjoy at room temperature or warmed in the microwave (the crust will become somewhat soft) or oven (best option).
TO FREEZE: Freeze baked, cooled tarts for up to 3 months, tightly wrapped in a double layer of plastic wrap or aluminum foil. Thaw overnight in the refrigerator and allow to come to room temperature prior to serving.