In a large bowl, sift together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt.
2 cups flour, 1 tsp baking powder, 1/2 tsp baking soda, 1/4 tsp salt, 1/4 tsp nutmeg, 1 tsp cinnamon
In a separate large bowl, whisk together the pumpkin purée, brown sugar, oil, sugar, and eggs. It'll be smooth.
3/4 cups brown sugar, 1/2 cup oil, 1 cup pumpkin puree, 1/2 cup sour cream, 2 eggs
Combine the dry ingredients with the wet and fold to create a smooth batter. Do not overmix!
Spoon half of the batter into the prepared pan and spread to cover the entire bottom of the pan. Sprinkle half of the streusel on top. Place the remaining batter on top and spread evenly. Finish off with the rest of the streusel.
Bake at 350°F for 35 minutes. Once a toothpick inserted comes out clean with a few crumbs, it is ready.
Slightly open your oven door and allow the hot air to escape for 10 minutes. Then remove from oven and let the cake sit in the hot pan for 10 more minutes. Then, grab the parchment paper to take it out of the pan and cool on a cooling rack.