Preheat the oven to 350 degrees. Liberally spray a tube pan with non-stick spray and set aside.

In a medium bowl, whisk together the gluten free flour, baking powder, baking soda, and salt. Set aside.

Add the butter and sugar to the bowl of a stand mixer fitted with the paddle attachment. Cream the butter and sugar together until light and fluffy (about 4 minutes). Scrape down the bowl as necessary.

Once the butter and sugar are creamed, add the the eggs one at a time. Mix until each egg is fully incorporated before adding the next egg. Add the sour cream and vanilla extract and mix to combine.

Add half of the dry ingredients and mix until almost combined. Then add the remaining dry ingredients and mix until fully incorporated, scraping down the bowl as needed.

Let the batter rest and make the streusel. Add the light brown sugar, 1/2 cup gluten free flour, cinnamon, and 1/4 teaspoon fine sea salt to a small bowl. Mix to combine.

Add the cubed butter and use a fork to to incorporate the butter until a crumble forms. Mix in the walnuts.

Transfer half of the coffee cake batter to the prepared tube pan and spread it evenly in the bottom of the pan. Sprinkle half of the streusel over the batter.

Add the remaining batter to the pan and spread it evenly over the streusel. Top with the remaining streusel.

Bake the coffee cake on the center rack for 50-65 minutes or until a cake tester comes out with moist crumbs.

Cool on a cooling rack for 30 minutes. Then remove the cake from the pan and cool completely.

Before serving, carefully transfer the coffee cake to a cake stand. Mix the powdered sugar, maple syrup, and vanilla extract together in a small bowl and drizzle over the top of the coffee cake.