Brown the butter:
In a light-bottomed saucepan over medium heat, melt the butter, stirring occasionally with a heatproof spatula. As it melts, the butter will begin to bubble gently and then more wildly. Keep stirring, scraping along the bottom to prevent burning. Within a few minutes, the bubbles will grow smaller, the foam will turn golden, and you’ll notice a rich, nutty aroma. As soon as you start to see brown specks at the bottom, remove from heat immediately. It can burn fast from here. Set aside to cool slightly.
Make the streusel topping: In a small bowl, whisk together the flour, sugar, cinnamon, and pumpkin pie spice. Pour in the brown butter and mix with a fork until crumbs form in the texture of damp-wet sand. Chill the streusel while you prepare the batter.
Prep your pan and oven: Preheat the oven to 350°F. Line a 8.5x4.5 in (or 9x5in) loaf pan with parchment paper, leaving overhang for easy removal.
Mix the dry ingredients: Combine the flour, baking powder, baking soda, pumpkin pie spice, cinnamon, and salt in a medium bowl. Set aside.
Mix the wet ingredients: In a medium or large bowl, add the oil and sugars and whisk to combine. Add the pumpkin puree, eggs, and vanilla and whisk to incorporate until smooth and well blended.
Combine the batter: Fold wet and dry mixtures together using a spatula, mixing only until just combined. Do not overmix. Add about ½ cup of batter to the bottom of the baking pan in order to avoid all the blueberries from falling to the bottom.
Add the blueberries: To the remaining batter in the bowl, fold in berries gently so they stay suspended and don’t stain the batter. If using frozen blueberries, use them straight from the freezer. Transfer the blueberry batter into the loaf pan, then top with streusel bycattering the chilled streusel evenly over the top.
Bake and serve: Bake until golden and set, about 1 hour 15 minutes in a 8.5x4.5 (about an hour in a 9x5 pan), or until a toothpick inserted into the center comes out clean. If the top browns too quickly, tent with foil. Let the loaf cool fully in the pan for 15 minutes, then lift out and cool completely on a rack. This ensures clean slices and proper texture.