• Preheat the oven to 350°F. Spray a 9x13-inch baking dish with cooking spray and set aside.

Prep the Streusel Layer:

• Place the butter in a medium microwave-safe mixing bowl and melt it in the microwave. Let it set for 5 minutes to cool slightly before using.
• Add the brown sugar, cinnamon, and flour to the melted butter. Stir them in until well combined and then set aside.

Prep the Cake:

• Add the softened butter, sugar, and brown sugar to the bowl of an electric mixer fitted with the paddle attachment. Beat them together until light and fluffy. Scrape the sides of the bowl.
• Add the sourdough starter, milk, sour cream, eggs, and vanilla to the mixer. Mix them in until well combined.
• In a separate large mixing bowl, whisk together the flour, baking soda, baking powder, and kosher salt.
• With the mixer on low, slowly add the flour mixture to the wet batter. Mix until just combined, it is ok to see some dry streaks of flour in the batter, you don't want to over mix it.
• Add the blueberries to a small bowl. Sprinkle the remaining 2 tablespoons of flour over the top and toss to coat. This helps prevent them from sinking in the cake.
• Pour half of the prepared cake batter into the prepared baking dish and smooth it into an even layer. Sprinkle half of the blueberries evenly over the top and then top with half of the streusel layer.
• Drop the remaining half of the cake batter over the top in large spoonfuls. Gently spread it evenly. Press the remaining blueberries into the top half of the batter and smooth over the top to even things out. Sprinkle the remaining streusel crumbs over the top.
• Bake the coffee cake for 40 - 50 minutes or until the cake has turned golden brown around the edges and the center is set. Let rest for 10 minutes before serving so the cake can firm up for better slicing.