Preheat the oven to 350°F (177°C) and line muffin tins with paper liners.
Whisk together the flour, baking powder, baking soda, salt, nutmeg, and ginger in a medium bowl to evenly distribute the leavening and spices.
3 cups (375 g) all-purpose flour, 1 tbsp (14 g) baking powder, 1 tsp (5 g) baking soda, ¼ tsp (1 g) table salt, ¼ tsp (0.5 g) ground nutmeg, ⅛ tsp (0.25 g) ground ginger
In a large bowl, beat the butter and sugar until light and fluffy, about 3 minutes. That air you build in now helps the muffins rise. Add the eggs one at a time, then mix in the pumpkin purée, buttermilk, and vanilla until smooth.
½ cup (113 g) butter, 1 cup (200 g) granulated sugar, 2 large eggs, 1 can (15 oz / 425 g) pumpkin puree, ¼ cup (60 ml) buttermilk, ½ tsp (2.5 ml) vanilla extract
Add the dry ingredients to the wet and mix just until combined. Overmixing knocks out the air you’ve already built, so stop as soon as the batter looks uniform. The batter should be thick, which gives the muffins their bakery-style domes.
Spoon the batter into the muffin cups, filling each about two-thirds full.
For the streusel, combine the sugar, flour, pumpkin pie spice, and cold butter in a small bowl. Use your fingers to rub the butter into the dry ingredients until the texture feels like damp sand with a few clumps. Sprinkle it evenly over each muffin.
½ cup (100 g) granulated sugar, ¼ cup (30 g) all-purpose flour, 1 tsp (3 g) pumpkin pie spice, 3 tbsps (42 g) cold butter
Bake on the center rack for 15 to 22 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the tin for 10 minutes, then transfer to a wire rack. They’re great warm, but the texture settles as they cool.