For the streusel:
Combine all of the ingredients except butter in a large bowl.
Sprinkle the butter into the bowl and rub into the dry ingredients until a crumbly texture forms with pieces ranging in size from small peas to almonds. Store in the refrigerator until ready to use.
For the chocolate pumpkin bread:
Preheat the oven to 350°F with a rack in the center of the oven. Spray and line an 8 1/2x4-inch loaf pan with parchment paper, then spray again.
Combine the butter and brown sugar in the bowl of a KitchenAid. With the paddle attachment, cream the butter and sugar together until lighter in color and increased in volume, about 2–3 minutes.
Scrape down the sides of the bowl (unless you’re using a Beater Blade !), add one egg and beat until completely incorporated. Scrape down the sides again, add the second egg and beat until incorporated.
Add the pumpkin purée and beat until mixture is completely uniform.
In a medium bowl, combine flours and mix well. Set aside 1/4 cup (30g) of the flour mixture. Then, add the baking soda, baking powder, cinnamon, ginger, cardamom, and salt to the bowl with the remaining flour.
Add the flour and spice mixture to the wet ingredients and fold until no dry patches are left.
Divide the batter into two portions. Fold the reserved 1/4 cup (30g) of flour into one portion and 1/4 cup (25g) cacao powder into the other portion.
Alternating the batters, scoop or spoon 2-3 tablespoon portions into the prepared loaf pan. Drag the blade of a paring knife or skewer through the batter to swirl and partially combine the scoops of batter. Sprinkle the streusel over the top and press firmly into the batter.
Bake until the top of the loaf has slightly browned and a metal or wooden skewer can be cleanly inserted into the middle of the loaf, about 45 – 50 minutes. Remove from the oven and allow to cool completely before removing from the pan.