Place a stick of butter in a small saucepan on medium-high heat. Stir continuously until you have a golden, foamy butter. Remove from heat and set aside to cool.
In a large bowl, combine the cooled brown butter, brown sugar, sour cream, eggs, and pumpkin puree. Mix until everything is well combined.
To your wet batter, add the flour, spices, salt, baking powder, and baking soda, and fold until everything is combined. Set aside.
In a small bowl, add the flour, brown sugar, cinnamon, salt, and butter. Work the dry ingredients into the butter until you have a crumble mixture that resembles clumpy sand.
Prepare an 8x8 baking pan with parchment paper or non-stick spray. Place half of the batter into the pan and add a thick layer of streusel. Add the remaining batter over the streusel and top off with the remaining streusel.
Bake at 375 degrees F for 45-60 minutes, until a knife from the center comes out dry.