Preheat your oven to 350°F. Grease or line a 9-inch square baking pan and set it aside.
Make the crumble.
In a medium-size bowl, place all of the crumble ingredients in the order listed, mixing to combine after each addition. You will have a thickly pourable mixture. Cover the bowl and place it in the refrigerator to chill until firm.
Make the cake.
In a large bowl, place the flour, xanthan gum, baking powder, baking soda, salt, pumpkin pie spice and granulated sugar, and whisk to combine well. Add the brown sugar and whisk again, breaking up any lumps in the brown sugar.
Create a well in the center of the dry ingredients and add the pumpkin butter, butter and eggs, mixing to combine after each addition. The batter will be thick.
Scrape the batter into the prepared pan and spread into an even layer.
Take the crumble topping from the refrigerator and, using the tines of a fork, break the topping up into irregular crumble pieces. Scatter the crumble evenly over the top of the cake and press down gently to help the topping adhere to the cake batter.
Place the cake in the center of the preheated oven and bake until a toothpick inserted in the center comes out with no more than a few moist crumbs attached, about 40 minutes.
Remove from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.