Preheat oven to 375 degrees. Cream together butter and sugars until creamy. Beat in pumpkin, buttermilk, eggs, molasses, and orange peel. Combine the dry ingredients in a small bowl and mix into the batter. Stir just until combined. Fill greased or paper-lined muffin tins 2/3 full of batter.

For the topping, combine the flour and brown sugar and oats (if using) in a small bowl. Cut in the butter until the mixture is crumbly. Spoon streusel topping over the muffins and bake for 20 to 25 minutes or until a tester inserted in the center comes out clean. Cool in the pan for 5 minutes before removing to a wire rack. Enjoy!