Cake Mix Sour Cream Coffee Cake

Preheat oven to 350°F. Grease an 8" or 9" square pan and line with a parchment sling. 

To make the crumb topping: In a medium bowl, add the 1 cup (156g) of cake mix, cinnamon, and chopped nuts. Then add the melted butter, stirring to form soft crumbs. Set aside. 

To make the cake: Place the remaining cake mix into the bowl of a stand mixer fitted with the flat beater attachment. Mix in the baking soda, then add the softened butter and beat on low speed to form small, uniform crumbs, 1 to 2 minutes. Beat in the eggs one at a time, scraping down the sides and bottom of the bowl as needed; you will have a smooth, thick batter. Scrape down the bowl, add the sour cream, and beat until no white streaks remain, about 30 seconds.  

Spread the batter into the greased pan then sprinkle the blueberries over the batter.

Distribute the crumb topping evenly over the batter, squeezing to form some larger crumbs.  

Bake for 35 to 40 minutes in an 8" square or 32 to 38 minutes in a 9" square, or until a toothpick inserted into the center comes out clean or with a few moist crumbs sticking to it. If using blueberries, increase the baking time by an additional 10 to 12 minutes. 

Remove the coffee cake from the oven and let it cool for 15 minutes on a wire rack before serving from the pan or using the parchment sling to transfer it to a cutting board or plate. Dust with confectioners’ sugar or non-melting sugar, if desired. 

Storage information: Wrap the completely cooled coffee cake airtight and store at room temperature for several days: freeze for longer storage.