In a medium-sized bowl, mix together the gluten free flour, xanthan gum, icing sugar and salt until combined.
Add the cold unsalted butter into the bowl. Work the butter into the mixture with your hands by pinching the cubes together with the flour mixture until it begins to resemble sand.
Add the water 1 tablespoon at a time into the bowl. Mix between each addition using your hands or a spatula. The dough starts off crumbly, then becomes sticky. Once all the ingredients are incorporated, form a ball with the dough and flatten it into a disc.
Wrap the dough with plastic or place in an airtight container and chill in the fridge for at least 30 minutes.
After chilling the dough, preheat the oven to 375°F.
Remove the dough disc from the fridge and place on the counter for a few moments to warm up. If the dough was chilled overnight, it may need some additional time, or a bit of kneading with warm hands to soften up slightly. Using a rolling pin, roll the dough out to ⅛” (3mm) thickness on a lightly floured surface.
Using a 3 ½” (9cm) circular cookie cutter, cut the dough into rounds. Place the dough inside the well on a standard muffin pan and lightly press it to fit properly. (Some overlap on the edges will happen, which is normal.) Be sure not to press the dough too firmly in the pan to avoid any sticking to the pan.
In a second medium-sized bowl, whisk together the brown sugar, maple syrup, eggs, vanilla extract, and salt. Add in any optional add-ins (raisins, nuts, or chocolate chips) and mix gently to incorporate.
Fill each muffin well lined with dough until they are ⅔ full. (The crust will expand as it bakes and the tarts will look full once done.)
Bake butter tarts in the oven for approximately 15-18 minutes until the crust appears set and the top has caramelized.
Gently remove the tarts from the muffin pan and place on a wire rack to cool. (This prevents any sticking if the filling happens to leak out.)
