Preheat the oven to 350°F (176°C). Grease a 9x9 baking pan with cooking spray or butter.
In a large mixing bowl, whisk together the flour, pumpkin pie spice, baking soda, baking powder, and salt.
In a medium mixing bowl, whisk the oil, granulated sugar, and light brown sugar until smooth. Add the eggs, sourdough discard, vanilla, pumpkin puree, and milk and whisk together until fully combined.
Add the wet ingredients to the dry ingredients, stirring until fully incorporated, making sure not to over-mix the batter.
Pour half of the cake batter into the greased baking dish. Sprinkle the pumpkin pie spice sugar mixture evenly over the top. Gently pour the remaining batter over the sugar layer, then spread it out evenly with an offset spatula. Crumble the topping with your fingers and sprinkle it evenly over top.
Bake for 40-45 minutes or until a toothpick inserted in the middle comes out clean.