Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking dish with cooking spray.
In a mixing bowl, whisk together the flour, granulated sugar, baking soda, baking powder, salt, cinnamon, and nutmeg.
In a separate bowl, combine the pumpkin puree, vegetable oil, eggs, and vanilla extract. Mix until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring just until combined. Do not overmix.
Pour the batter into the prepared baking dish and spread it evenly.
In another small bowl, beat the softened cream cheese with the powdered sugar until smooth. Drop spoonfuls of the cream cheese mixture over the pumpkin batter. Use a knife to swirl it gently for a marbled effect.
In a separate bowl, combine the chopped nuts, brown sugar, and melted butter. Sprinkle this mixture evenly over the top of the cake.
Bake in the preheated oven for 30 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan before slicing into pieces and serving.
Serve warm or chilled with a dollop of whipped cream for added richness.
Store any leftovers in an airtight container in the refrigerator for up to 4 days.
You can substitute the nuts with crushed graham crackers if you prefer a different topping.