Prepare the Cinnamon Sugar Filling: In a medium bowl, combine 1½ cups of brown sugar, 5 tablespoons of all-purpose flour, and 4 teaspoons of ground cinnamon. Stir in 6 tablespoons of melted butter until the mixture is well combined and crumbly.

Preheat Oven: Preheat your oven to 375°F (190°C) and gather your 9x13 inch baking pan, greasing it with butter or non-stick spray.

Mix the Cake Batter: In a large mixing bowl, cream together ½ cup of softened salted butter and 1½ cups of white sugar until light and fluffy. Add 2 teaspoons of vanilla extract, 2 eggs, 1 cup of whole milk, and 1 cup of sourdough discard, and mix until smooth. Gently incorporate 4 teaspoons of baking powder and 3 cups of all-purpose flour.

Prepare Blueberries: Toss 1½ cups of fresh blueberries in 1 tablespoon of flour to prevent sinking. Fold most of the blueberries into the batter, reserving a handful for topping.

Layer in Baking Pan: Pour half of the batter into the pan, layer with half of the cinnamon sugar filling, spread the remaining batter, layer with the rest of the filling, and scatter reserved blueberries on top.

Bake: Bake for 40-45 minutes until the cake is golden brown and a toothpick comes out clean. Let cool in the pan for about 10-15 minutes before slicing.