Preheat the oven to 400°F. Prepare a 12-cup metal muffin tin by greasing 10 cups with coconut oil or cooking spray.
Pull the tart dough out of the freezer. If it is too solid, allow to sit at room temperature for a few minutes. Once you're able to slightly poke the dough, remove from the plastic wrap. It's time to work, and work quickly.
Place the dough disk between 2 new pieces of plastic wrap or parchment paper. Be sure to sprinkle both pieces with some extra gluten-free flour to avoid sticking. The plastic or parchment will help with removing the tart cut outs easily. Use a rolling pin on top of the covered dough to roll to a thickness of approximately ⅛". Use a 3.5" diameter cookie cutter (or a glass) to cut out tart rounds. If it's a bit sticky, dust both sides of the round with flour.
Gently push the round into one of the greased muffin cups. It may fold in on itself slightly and that's okay. Use your fingers to gently repair and form a little cup shape. Use a fork to prick the bottom of the tart twice for air release when baking. Repeat with the rest of the tart dough. You should produce about 10 butter tarts. You may have to re-roll the dough in order to make the cut-outs. If the dough begins to melt and become sticky, wrap it in the plastic wrap again and place in the freezer for 10 minutes before continuing. While you wait for the dough to re-solidify, place the muffin tin in the fridge to chill as well. Once all of the dough is cut and placed into the muffin tin, bake at 400°F for 8-9 minutes.
Remove the muffin tin from the oven and drop the temperature to 375°F. Place about 3 sliced pecans (or a handful of whatever fillings you like) into each tart shell. Pour the filling into each tart shell to just below the top edge. Don't go too high or you risk the filling bubbling over. Bake again at the lower temperature for 10-11 minutes until the crust is golden brown.
Remove from the oven and allow to cool in the pan for about 5 minutes. Run a sharp knife around the edge of the tart shells to gently remove the butter tarts. Transfer to a wire rack to cool. Enjoy warm after about 15 minutes, or once they come to room temperature. Once cooled completely (approximately 2 hours), store in an airtight container at room temperature for up to 3 days. Some people also like to enjoy them cool from the fridge! Butter tarts will keep in the fridge for 5 days. You can also freeze butter tarts for up to 1 month, and allow to thaw at room temperature for a few hours before enjoying again.