Preheat oven to 350 degrees. Cream butter in a large bowl. Beat in sugar and vanilla until light and fluffy. Add flour and pecans and mix well. Chill dough in tightly covered container overnight. Remove the foil from the Hershey's Kisses. Shape dough around kiss using about a teaspoon of dough per Kiss. Bake 10-12 minutes on ungreased cookie sheet until set, but not brown. While warm, sprinkle with powered sugar. Store in a covered container.