Preheat the oven to 350°F (180°C).For the tart shells, add the butter and cream cheese to the Deluxe Stand Mixer and set to BEAT for 1 minute. Add the flour and select MIX for 1 minute and 30 seconds. The dough will be soft and smooth.Shape the dough into 1” (2.5-cm) balls. Place the dough balls into the ungreased cups of the Mini Muffin Pan. Press your thumb, or a small spoon coated in flour, into the center of each dough ball to create a divot. Using your thumb or the spoon, push the dough up the sides of the mini muffin pan until the dough rises slightly above the rim of the pan.For the filling, place the butter in a small microwavable bowl; microwave on HIGH for 30 seconds or until melted. Stir in the brown sugar, egg, and vanilla. Finely chop the pecans; add them to the bowl and mix well.Fill each tart shell with a level tablespoon (15 mL) of the filling (do not overfill). Bake for 20-25 minutes or until the cookies are light golden brown. Remove the pan from the oven, and let the cookies cool in the pan for 3 minutes. Remove the cookies from the pan to a cooling rack and let them cool completely. Sprinkle with powdered sugar or cinnamon, if desired.