Gluten Free (Or Not) Sour Cream Cinnamon Coffee Cake

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This one is a keeper, and it doesn't taste gluten free. I promise. 

Adapted from Baked: New Frontiers in Baking, I used our gluten free flour mix that comes from Gluten-Free Baking Classics. Combine a delicious recipe from a renowned bakery and a really good gluten free flour mix? Yes!
 

Flour Mix:
2 parts brown or white rice flour
2/3 parts potato starch
1/3 part tapioca flour

Of course, you can make it with regular all-purpose flour, too. Just omit the xantham gum and substitute the same amount of all-purpose flour for the gluten free mixture.

Sour Cream Cinnamon Coffee Cake (With a Dark Chocolate Swirl) 

Crumb Topping
3/4 cup gluten free flour mix (above)
3/4 cup firmly packed dark brown sugar
1/2 t salt
6 T butter, cold, cut into 1" cubes

Chocolate Cinnamon Swirl
1/2 c sugar
1 t dark unsweetened cocoa powder (I use King Arthur's Black Cocoa)
1 t cinnamon

Sour Cream Cake
3 1/2 c gluten free flour mix
1 t xantham gum
1 t baking powder
1 1/2 t baking soda
1/2 t salt
1 c butter, soft but cool
2 1/4 c sugar
4 large eggs
16 oz sour cream
1 1/2 t vanilla extract

To make the crumb topping, pulse the flour, sugar, and salt in a food processor for 5 seconds to mix. Add the butter and pulse until combined. The mixture will look like crumbly, course sand. Set aside in the fridge.

To make the chocolate cinnamon swirl, whisk together the sugar, cocoa powder, and cinnamon. Set aside.

To make the sour cream cake, preheat oven to 350°. Butter a 9x13 baking pan. In the bowl of an electric mixer, cream the butter thoroughly. Add sugar and beat until fluffy. Add the eggs, one at a time, beating well after each addition. Scrape down the bowl and mix for 30 more seconds. 

Add the sour cream and vanilla and beat until incorporated. Add the dry ingredients in three additions, scraping down the bowl in between each addition. Don't overmix. 

Pour 1/3 of the batter into the prepared 9x13 and spread evenly. Sprinkle half the chocolate cinnamon swirl mix over the batter, covering the entire surface of the batter. Spoon another 1/3 of the batter over the swirl and spread evenly. Top with remaining swirl mixture, and then spread the last of the batter on top. Sprinkle the crumb mixture evenly over. 

Bake for 1 hour, or until a toothpick stuck into the middle comes out clean (crumbs are okay; no gooeyness). Let the cake cool on a rack for 30 minutes before slicing.