Preheat oven to 350°F. Spray 12-cup muffin tin with non-stick cooking spray or line with paper muffin liners.
Remove ¼ cup cake mix from package; set aside. Combine walnuts, sugar and cinnamon in a small bowl; set aside ¼ cup. Mix remaining walnut mixture with reserved cake mix and 2 tbsp. butter. Stir until mixture forms a crumbly streusel; set aside.
Beat cake mix, 4 tbsp. butter and eggs in the bowl of an electric mixer on medium for 2 minutes. Spoon a heaping tablespoon of batter into each cup of the muffin pan. Top each muffin with 1 tsp. of the reserved walnut mixture. Divide the remaining muffin batter among the cups. Top with reserved streusel mixture.
Bake for 20 minutes or until a toothpick inserted into the center of each muffin comes out clean. Cool 5 minutes before removing from pan.