For filling, place butter inSmall Batter Bowl; microwave on HIGH 30 seconds or until melted. Stir in brown sugar, egg and vanilla. Finely chop pecans usingFood Chopper; add to batter bowl and mix well.
Using Small Scoop, fill each tart shell with level scoop of filling (do not overfill). Bake 20-25 minutes or until light golden brown. Remove from oven; cool in pan 3 minutes. Remove from pan toStackable Cooling Rack; cool completely. Sprinkle with powdered sugar, if desired.