Preheat the oven to 350°F (180°C) and spray two standard muffin tins with nonstick spray.
cooking spray
To make the crumble, combine the flour, melted butter, brown sugar, chopped walnuts, and pumpkin pie spice in a small bowl. Mix until the butter is fully absorbed and the mixture forms clumps that hold together when pressed. Set aside while you make the batter.
¼ cups (30 g) all-purpose flour, 3 tbsps (42 g) butter, ¼ cup (50 g) packed light brown sugar, ½ cup (60 g) chopped walnuts, ½ tsp (0.5 g) pumpkin pie spice
In a medium bowl, whisk together the flour, baking powder, baking soda, and pumpkin pie spice. This step helps distribute the leavening evenly.
2 cups (240 g) all-purpose flour, ½ tsp (2.5 g) baking powder, 1 tsp (5 g) baking soda, 1 tbsp (5 g) pumpkin pie spice
In a large mixing bowl, beat the softened butter with the granulated sugar and brown sugar until light and fluffy, about three minutes. Creaming properly here gives the muffins lift and structure.
¾ cup (170 g) butter, ¾ cup (150 g) granulated sugar, ¾ cup (150g) packed light brown sugar
Add the eggs one at a time, mixing just until each is incorporated. Scrape down the sides of the bowl, then mix in the pumpkin until smooth and consistent.
2 large eggs, 15 oz (425 g) pumpkin puree
Gradually add the dry ingredients to the pumpkin mixture, mixing on low speed until no streaks of flour remain. Stop as soon as the batter comes together to keep the muffins tender.
Scoop just under a ½ cup (120 ml) of batter into each prepared muffin cavity, filling them just below the top. Avoid overfilling, as the batter will rise and can bake over the edge. If you prefer, line the tins with paper liners, which work well for larger muffins. Evenly divide the crumble mixture over each muffin, pressing it lightly so it adheres. If any cavities remain empty, fill them halfway with water to promote even baking.
Bake for 25 to 30 minutes, or until the tops are golden and a toothpick inserted in the center comes out clean. Let the muffins cool in the pan for 15 to 20 minutes before transferring them to a wire rack. The crumble sets as they cool, so let them rest before removing.