In a medium bowl, mix together the brown sugar, flour, pecans, and pumpkin pie spice. Add the melted butter and stir with a fork until the mixture becomes crumbly. Set aside.
½ cup brown sugar, ¼ cup all-purpose flour, ½ cup finely chopped pecans, 1 teaspoon pumpkin pie spice, 3 tablespoons unsalted butter
Preheat oven to 350°F (180°C). Lightly grease the bottom of an 8-inch square baking pan.
In a large bowl, combine flour, baking powder, baking soda, salt, pumpkin pie spice, brown sugar, and granulated sugar.
2 cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon fine sea salt, 1 ½ teaspoons pumpkin pie spice, ½ cup brown sugar, ½ cup granulated sugar
In a separate bowl, whisk together pumpkin puree, vegetable oil, sour cream, and eggs.
1 cup pumpkin puree, ½ cup vegetable oil, 4 ounces sour cream, 2 large eggs
Make a well in the center of the dry mixture, then pour in the wet ingredients. Stir gently until just combined—do not overmix.
Spread half of the batter into the prepared pan. Sprinkle with half of the streusel. Add the remaining batter, then top with the rest of the streusel.
Bake for about 45 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 15 minutes.
In a small bowl, whisk powdered sugar with milk until smooth and pourable.
½ cup powdered sugar, 2 tablespoons milk
Drizzle the glaze over the warm cake before serving.