Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, pumpkin pie spice, and salt until fully combined.
In a separate bowl, mix together the pumpkin puree, vegetable oil, eggs, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined (do not overmix).
Pour the batter into the prepared cake pan, spreading it evenly.
For the crumble topping, combine rolled oats, flour, brown sugar, and cinnamon in a medium bowl. Add cold butter and use your fingers or a pastry cutter to blend until crumbly.
Sprinkle the crumble mixture evenly over the cake batter in the pan.
Bake in the preheated oven for 35-40 minutes or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
Notes
Dust the cooled cake with powdered sugar and serve with a dollop of whipped cream on top, garnished with a sprinkle of cinnamon.