Cream butter in a medium mixing bowl; gradually add sugar, beating until light and fluffy. Add vanilla, beating well. Add flour and cocoa to creamed mixture, stirring until well blended. Fold in pecans.
Shape into 1/2-inch balls; place on lightly greased cookie sheets. Bake at 350° for 12 to 15 minutes; cool slightly on cookie sheets. Remove to wire racks to cool completely. Roll in powdered sugar.