Pumpkin Streusel Muffins

Preheat the oven to 350 degrees F (175 degrees C). Grease the bottoms only of three 12-cup muffin tins with cooking spray.

Combine oats, walnuts, brown sugar, and flour in a bowl. Cut in cold butter with a pastry blender or fork until the mixture resembles coarse crumbs. Place streusel topping in the refrigerator until ready to use.

Stir pumpkin puree, sugar, oil, eggs, and vanilla extract together in a large bowl. Add flour, baking soda, salt, cinnamon, baking powder, cloves, ginger, allspice, and cardamom; mix until combined. Fold in walnuts. Pour batter into the prepared muffin cups, filling each 1/2 full. Evenly distribute streusel topping on top of each muffin.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 25 minutes. Cool in the tins for 5 minutes. Transfer to a wire rack to cool completely.