Brown the butter:

1. Add 1 cup of butter into a large skillet. Cook over medium heat while whisking until the butter melts, bubbles, and eventually browns. This will take about 5-7 minutes start to finish.
2. Once the butter is browned, immediately pour it into a measuring cup - make sure to scrape in the brown bits from the bottom of the pan! It will measure slightly under a cup after browning.
3. While the butter is still hot, mix in 1-2 tablespoons of cool butter to bring it back up to 1 cup. You may need slightly more or slightly less depending on the brand of butter you used.
4. Set the butter aside until it cools to room temperature but is still liquid (about 15 minutes).

Pumpkin spice streusel:

1. Preheat your oven to 325F. Grease an 8x8" baking pan and line the bottom and sides with parchment paper. Set aside.
2. In a medium bowl, mix together all the ingredients for the pumpkin spice streusel minus the cocoa (flour, both sugars, and pumpkin spice).
3. Back to the cup of brown butter, the browned bits will have settled in the bottom of the cup. So give it a little mix to redistribute them then pour 1/2 cup of the browned butter into the bowl with the dry ingredients for the streusel.
4. Mix together until moistened and crumbly.
5. For the center swirl, scoop out 1/3 cup of the streusel and add it into a small bowl with the cocoa powder. Mix together to combine.
6. Place the two streusel mixtures into the fridge while you work on the cake batter.

Pumpkin cake:

1. In a large bowl, combine the remaining half cup of brown butter, pumpkin puree, granulated sugar, brown sugar, egg, sour cream, and vanilla. Whisk to combine.
2. Mix in the baking soda, pumpkin spice, and salt. Lastly, mix in the flour until you have a streak free batter. Take care not to overmix. The batter will be a bit thick.
3. Pour half of the pumpkin cake batter into your prepared pan and spread into an even layer. Take the cocoa streusel mixture out of the fridge. It will have hardened slightly, so use a fork to break it up into tiny pieces and sprinkle it evenly on top of the layer of cake batter.
4. Dollop the remaining cake batter over the streusel. It's best to do little dollops to try to get it as evenly covered as possible and then use a spoon or spatula to lightly smoothen the top so you don't disturb the streusel underneath. Finally, take the regular streusel out of the fridge. Again, use a fork to break it up into smaller clumps then sprinkle it on top of the layered cake batter.
5. Bake the cake in the center of your preheated oven for 55-70 minutes or until evenly puffed and a skewer inserted into the center comes out clean with a few moist cake crumbs.
6. Cool the cake in the pan until it's no longer hot. Dust some powdered sugar on top before slicing and serving!