In a mixing bowl, cream butter, shortening and sugar. Beat in eggs, one at a time. Add extracts. Combine flour and baking powder; add to creamed mixture and mix well. Spread two-thirds of the batter in a greased 15x10x1 inch baking pan. For filling, combine flour, cornstarch and tapioca in a large bowl. Add 1/2 cup of blueberries; mash with a fork and stir well. Add lemon peel and remaining berries; toss to coat. Pour evenly over batter in pan. Drop remaining batter by rounded tablespoonfuls over filling. Bake at 350° for 40 minutes or until golden brown. Combine glaze ingredients; dirzzle over warm cake.