Preheat oven to 350°. In a medium bowl, stir together the brown sugar, chocolate pieces, flour, baking powder, salt, pecans and toffee pieces. Set aside. In a large bowl, combine butter, eggs and vanilla. Stir brown sugar mixture into butter mixture. Spread batter into a greased 8x8x2-inch baking pan.
Bake for 25 to 30 minutes or until set and golden brown and edges just begin to pull away from pan. Cool in pan on a wire rack. Cut into bars.