Preheat the oven to 350° F. Line a muffin pan with cupcake liners. This recipe makes 18 muffins, so prepare a second muffin pan in the same manner. Set aside.
In a large bowl, whisk together the flour, baking soda, baking powder, and pumpkin pie spice. Set aside.
In a separate medium bowl, whisk together the melted butter, brown sugar, pumpkin puree, vanilla, buttermilk, and vinegar.
Add the wet ingredients to the dry ingredients and mix just until combined. Spoon the batter into liners, filling 2/3 of the way full.
Make crumble: add the sugar, flour, and cinnamon to a medium bowl and stir to combine.
Add the butter and use a fork, or pastry cutter, to cut the butter into the flour until the mixture is made of coarse crumbs.
Sprinkle the mixture evenly over all of the muffins.
Bake for about 25 -30 minutes, or until a toothpick inserted in the center comes out clean.
When muffins are cool, whisk together the powdered sugar and milk and drizzle over the tops.