Preheat the oven to 350 degrees.
Make the crumble. Combine the dark brown sugar, flour, spices, and salt. Drizzle the butter over it, and use a fork to combine, forming nice large crumbles. Set aside.
Make the cake. Combine the flour, baking powder, baking soda, spices, and salt. Set aside.
Whisk together the pumpkin purée, molasses, and brown sugar. Add the eggs one by one, beating after each addition. Drizzle in the melted butter.
Fold the dry ingredients into the wet, stirring with a rubber spatula until just combined.
Pour the batter into a greased 9-inch spring form pan. Scatter the crumble over the top.
Bake for 25 minutes, or until a toothpick inserted into the center of the cake comes out clean, with just a few crumbs attached. Allow to cool about 15 minutes before unmolding and serving.