Line a standard 12-cup muffin pan with paper liners. Preheat the oven to 425°F.
Make the streusel: Rub together 60 grams flour, 65 grams coconut sugar, cinnamon, and cold diced butter until clumpy. Refrigerate while preparing the batter.
In a large bowl, whisk the remaining 240 grams flour with the baking powder, baking soda, salt, and pumpkin pie spice.
In a separate bowl, whisk the eggs, pumpkin puree, buttermilk, 180 grams coconut sugar, melted butter, and vanilla until smooth.
Fold the dry ingredients into the wet ingredients just until combined. Let the batter rest for 10 minutes to hydrate the freshly milled flour.
Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full. Distribute the chilled streusel generously over the tops.
Bake at 425°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F and bake for 15 minutes more. A toothpick inserted near the center should come out clean or with a few moist crumbs.
Cool the muffins in the pan for 5 minutes, then transfer them to a wire rack.