Meanwhile, in 2-quart saucepan, melt butter and peanut butter over medium heat, stirring occasionally. Remove from heat; stir in vanilla. Cool 1 minute. Stir in powdered sugar. When cool enough to handle, knead filling several times until powdered sugar is thoroughly blended. Shape into log, about 9 inches long and 2 inches in diameter. Cut into 18 (1/2-inch) slices, reshaping slices into round shape if necessary. Cover with plastic wrap; set aside.