Preheat oven to 375F. Line a muffin tin with paper liners. Set aside.
In a large bowl, cream the butter, granulated sugar and brown sugar together.
Mix in the pumpkin, sour cream, eggs and molasses.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt and pumpkin pie spice.
Add the flour mixture to the pumpkin mixture and stir until just combined.
Scoop the batter into the prepared muffin tin, filling the cups ¾ full.
In a small bowl make the topping by mixing together the flour and brown sugar then use a fork to cut in the butter until it is crumbly.
Sprinkle the topping generously over the top of the muffins.
Bake for 20-25 minutes, until a toothpick comes out clean.
Let cool for 5 minutes before transferring to a wire rack to cool completely.