Begin by melting the butter in a small microwave-safe bowl. Allow it to cool for a few minutes.

While the butter is cooling, combine gluten-free multipurpose flour, light brown sugar, granulated sugar, chopped walnuts, cinnamon, and kosher salt in a mixing bowl. Whisk these ingredients together.

Using a fork or a small spatula, gently blend the dry ingredients with the melted butter until crumbles start to form. Be careful not to overmix.

Place the walnut streusel topping in the refrigerator while you prepare the gluten-free cake batter for the Carrot Cake Coffee Cake.

Preheat the oven to 350F. Grease a 9" springform pan with nonstick spray and line the bottom with parchment paper.

Peel and grate the carrots using the smallest setting of your box grater. Set aside.

In a microwave-safe bowl melt the butter. Then combine it with the oil and allow for it to cool for a few minutes.

While the butter-oil mixture is cooling, combine the gluten free multi-purpose flour, cornstarch, baking powder, baking soda, the spices and kosher salt in a medium bowl. Make sure to sift or stir it together.

In a large bowl combine the light brown sugar, granulated sugar, sour cream, vanilla extract, room temperature large eggs, and the cooled butter-vegetable oil mixture. Whisk until smooth and fully combined.

Add the dry ingredients to the wet ingredients. Mix until the gluten-free carrot cake batter is smooth. Make sure not to overmix the batter which can cause the carrot cake to be dense.

With a flexible spatula, fold the grated carrots into the cake batter. Make sure they are evenly distributed.

Transfer the carrot coffee cake batter to the prepared spring form. With a small offset knife, spread it out evenly.

Top the cake batter with the walnut crumble topping. Make sure to gently press down the streusel topping with the back of a spoon or clean hands. This ensures it sticks to the cake.

Place the carrot cake in the preheated oven and bake at 350F for 65-70 minutes. The crumb top of the cake will be golden brown and a cake tester or toothpick insert in the center of the cake comes out clean.

Take the gluten-free crumb cake out of the oven and set it on a cooling rack. Let the cake rest in the pan for about 10 minutes before detaching the springform pan's ring.

Allow the cake to cool for at least 90 minutes on a wire rack before cutting and serving.