Beat butter at medium speed with an electric mixer until creamy; gradually add 1/2 C. sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition. Combine 3 3/4 C. flour and next 3 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Toss raspberries and blueberries with remaining 1/4 C. flour; gently fold into batter. Spoon batter into greased and floured muffin pans, filling three-fourths full. Sprinkle evenly with remaining 1/4 C. sugar. Bake at 400° for 20 to 25 minutes or until golden brown. Remove from pans immediately, and cool on wire racks. Yield: 2 dozen.