Mix butter, cream cheese and flour together until just combined. Roll the dough into small balls - about 1" in diameter and press each dough ball evenly into a small cup muffin tin, making sure dough comes up the sides of the muffin cup.
Mix pecans, brown sugar, egg, butter and vanilla together. Pour into pastry cups, coming not quite to the top of the cup and bake 20 minutes. Remove from oven and cool on a rack. Once cool, dust the tops of the nut cups with powdered sugar.