Cream butter in a large mixing bowl; gradually add brown sugar, beating well. Add vanilla; mix well. Gradually add flour, 1 cup at a time, stirring until well blended.
Shape dough into 1-inch balls, inserting one pecan half into center of each ball. Place cookies 2 inches apart on greased cookie sheets. Bake at 375° for 12 minutes or until lightly browned. Remove from cookie sheets, and cool on wire racks. Roll in powdered sugar.
Note: Cookies may be frozen before rolling in powdered sugar.