Remove cereal mixture from pan by lifting with foil; peel back foil. Using large sharp knife, cut mixture in half lengthwise into 2 rectangles. To mark 1 rectangle for cutting, place toothpick at each end of one long side; add 3 toothpicks to same side, about every 3 inches, dividing into fourths. On other long side, place toothpick 1 1/2 inches from each end; add 3 toothpicks to same side, about every 3 inches, dividing into thirds. Cut diagonally across rectangle from toothpick to toothpick, forming triangles. Press together 2 end pieces to form eighth triangle. Repeat with remaining rectangle.