Preheat your oven to 375°F and line a muffin pan with liners.
In a bowl, mix the pumpkin, melted butter, granulated sugar + brown sugar until smooth. Add the eggs, vanilla + buttermilk and mix until combined.
Add the flour, baking soda, baking powder, salt, cinnamon + nutmeg and gently fold until just combined. Don’t overmix!
Make the streusel by mixing the brown sugar, flour + cinnamon together, then use a fork or your hands to work in the softened butter until crumbly and pea-sized.
If your streusel starts turning into a dough instead of crumbs, pop it in the freezer for a few minutes. If it’s still too soft, slowly add 1 tbsp of flour at a time and freeze again until you get a crumbly consistency.
Fill each muffin cup about halfway with pumpkin batter, add a layer of streusel then cover with more pumpkin batter.
Generously top each muffin with the remaining streusel.
Bake at 375°F for 5 minutes, then without opening the oven, reduce the temperature to 350°F and continue baking for about 18–22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let them cool for a few minutes before transferring them to a cooling rack!
