Preheat the oven to 350°F. Grease an 8 or 9-inch square baking pan and line it with parchment.
Make the crumb topping: In a medium bowl, whisk the ½ cup gluten free flour, ½ cup brown sugar, and 1 teaspoon ground cinnamon together. Cut in the cold ¼ cup unsalted butter using a pastry blender or a fork. Mix to create clumps and crumbs. Place the bowl in the fridge to chill while you make the cake batter.
Make the cake: In a large bowl, whisk together 2 ¼ cups gluten free flour, 1 teaspoon baking soda, ½ teaspoon baking powder, ½ teaspoon fine sea salt, 1 ½ teaspoons ground cinnamon, ½ teaspoon ground nutmeg, ¼ teaspoon ground cloves, and ¼ teaspoon ground ginger until combined.
Now whisk and stir in the 1 cup pure pumpkin puree, 2 large eggs, ¾ cup granulated sugar, ½ cup oil, and ¼ cup whole milkuntil smooth and combined. Transfer the batter to the prepared baking pan and scatter the topping evenly across the surface. Let batter rest for 10-15 minutes.
Place in the oven and bake the cake for 35-45 minutes. Check at 35 minutes; if a toothpick comes out clean (a few moist crumbs are OK), take it out of the oven; otherwise, keep baking and checking every few minutes or so.
Cool the cake in the pan set on a wire rack for at least 20 minutes before serving.
Notes
CN's Tips
Chill the topping - This step is not strictly necessary, but I like to assemble the topping first, then place the bowl in the fridge while I make the cake batter. This helps the topping to firm up and hold its shape better during baking.
Rest the batter - Especially when using a gluten-free flour blend, it's important to rest the batter for at least 10 minutes to allow the flours to hydrate. This will create a smoother, more tender crumb after baking.
Pumpkin pie spice - Instead of the combination of ground spices, use 2 ½ teaspoons of pumpkin pie spice to streamline prep a bit.
Keep it dairy-free - Use non-dairy butter in the topping and non-dairy milk in the cake to make this cake easily dairy-free.
Add vanilla - Stir in 2 teaspoons of pure vanilla extract or vanilla bean paste to the batter.
Nuts - Sprinkle about ½ cup of sliced almonds into the topping mixture for a bit of crunch.