Roll the pie dough out about ⅛" thick. Use a cookie/biscuit cutter or drinking glass to cut the pastry into about 18 4-inch circles. Push the circles into the cups of two 12-cup muffin pans (or one 12-cup and one 6-cup pan, or make the tarts in two separate batches). Cover the muffin pan with plastic wrap or a clean, damp dish towel and refrigerate until ready to fill.
Maple Pecan Tarts
Move an oven rack to the middle position, and heat the oven to 400°F. Note that if you plan to cook two muffin pans at once, you can use the upper middle and lower middle positions.
Place 1 cup coarsely chopped pecans in a small skillet over medium heat. Toast the pecans, stirring often, until fragrant and golden brown. Make sure to keep an eye on them to prevent burning. Transfer the pecans to a heat-proof plate or bowl to cool.
Place ¼ cup unsalted butter in a small saucepan. Melt the butter over medium heat, about 3-5 minutes. Remove the saucepan from the heat and whisk in ½ cup pure maple syrup, 1 large egg, ¼ cup packed brown sugar, and ¼ teaspoon salt until uniform.
If using store-bought tart shells, set 15 of them onto a baking sheet. Distribute the toasted pecans evenly among the tart shells, placing them into the bottoms of the shells.
Pour the maple syrup mixture into the tart shells, making sure to distribute the mixture evenly. It's ok if the mixture doesn't reach the top of the tart shells, as they can overflow during baking if they are overfilled. Place the baking sheet/muffin pan(s) onto the middle (or upper-middle and lower-middle) rack(s). Bake for 12 minutes for homemade tart shells and/or runnier tarts, or up to 15 minutes for store-bought tart shells with fully set filling. Cool tarts completely before serving.
Store the tarts in an airtight container or zipper bag at room temperature for up to 2 days, or in the freezer for several weeks. Thaw at room temperature before serving.