Preheat oven to 375°. In a large bowl, cream butter and sugars until light and fluffy, about 5 minutes. Beat in the pumpkin, buttermilk, eggs, molasses and orange zest. In a second bowl, combine the flour, baking soda, baking powder, pumpkin pie spice and salt; gradually add to pumpkin mixture just until blended. Fill greased or paper-lined muffin cups two-thirds full. , For topping, combine flour and brown sugar; cut in butter until mixture is crumbly. Sprinkle over batter. Bake until a toothpick inserted in the muffin comes out clean, 20-25 minutes. Cool in pan for 5 minutes before removing to a wire rack to cool completely.