Using a food processor, pulse the 2-1/2 cups flour, 1 tablespoon sugar and 1 teaspoon fine sea salt. Add the 1 cup unsalted butter in chunks, pulse 5 to 8 times, or until butter is in small pieces. Add the 3/4 cup cold water and pulse until the dough starts coming together.

Turn the dough onto 2 plastic wrap pieces lying flat. Cover over and pat into a disc. Refrigerate for at least 1 hour. Dough will be super sticky, this is normal.

Sprinkle a small amount of flour on a flat surface and roll out one portion at the time, about 1/8 of an inch thick. Use a round cutter or lid (4 inches) and cut 15 pieces.

Grease 1 muffin pan completely and 3 muffin cups from a second pan. Press each dough circle into the bottom of a muffin pan cup. Refrigerate while making the filling.

Preheat the oven to 350°F.

Cream the softened ¼ cup unsalted butter and packed 1/2 cup brown sugar together. Add the 2 eggs one at the time. Mix in the 1 cup maple syrup, 1/4 teaspoon fine sea salt, and 1 teaspoon vanilla extract. The filling is a chunky custard and very loose. Allow the filling to sit for 5-10 minutes to let the sugars dissolved fully, then give it one more whisk.

If adding raisins or nuts, sprinkle into each muffin well before adding the filling. Pour the filling into pie crust and bake for 25 minutes. It is easiest to use a glass measuring cup with a pour spout.

Let the tarts cool for 5-10 minutes in the pan and remove to a cooling rack.