In a large bowl, whisk the egg, pumpkin, sugars, oil, sour cream, vanilla, and spices.
In another small bowl, combine the flour, baking powder, baking soda, and salt.
Fold dry ingredients into wet ingredients until just combined.
Pour 2/3 of the loaf batter into a baking loaf pan sprayed with baking spray.
Gently scoop the 2/3 of the cream cheese mixture on top and spread out to cover evenly. Swirl throughout with a large skewer.
Gently scoop the remaining 1/3 of the batter on top and spread to the edges, covering the cream cheese layer.
Add the remaining 1/3 of the cream cheese mixture, spreading over top.
Sprinkle with the walnuts then coat with the brown sugar.
Bake for 45-50 minutes at 375F (covering for last 15 with foil to prevent burning.
The loaf is done when a skewer inserted into the middle comes out clean.
Remove from oven and let cool in the pan for 15 minutes then turn onto a rack to completely cool.