Preheat Oven:
Preheat oven to 350°F (175°C). Grease or line an 8x8-inch baking pan with parchment paper.
Mix Dry Ingredients:
In a medium bowl, whisk together flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
Combine Wet Ingredients:
In a large bowl, whisk together pumpkin purée, brown sugar, granulated sugar, melted butter, eggs, and vanilla until smooth.
Make the Batter:
Gradually fold the dry ingredients into the wet mixture until just combined — avoid overmixing.
Prepare the Crumble:
In another bowl, combine flour, sugars, and cinnamon. Cut in cold butter using a fork or pastry cutter until the mixture is crumbly.
Assemble:
Spread the batter evenly into the prepared pan and sprinkle the crumble topping over the top.
Bake:
Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.
Cool and Glaze:
Let the cake cool for 15 minutes. Whisk together powdered sugar, milk, and vanilla, then drizzle over the cooled cake before serving.