Streusel: Combine brown sugar and cinnamon in a small bowl. Add butter and nuts. Set aside.
Cake: Combine cake mix, pumpkin, eggs, and 1/4 cup butter in a large mixing bowl. Beat at medium speed 3 minutes. Spread half of the batter into ungreased 13 in x 9 in x 2 in pan. Sprinkle half of streusel over batter. Spread remaining batter over streusel. Top with remaining streusel.
Bake 40 to 45 minutes or until toothpick test passes.