Preheat & Prep: Preheat the oven to 300°F. Spray a bundt pan with nonstick cooking spray and dust with flour, tapping out excess. Set aside.
Make the Streusel: In a small bowl, combine the granulated sugar, flour, cinnamon, butter, oats, and water. Mix until crumbly. Set aside.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
Cream Butter and Sugar: In a large mixing bowl or stand mixer, beat the butter and sugar until light and fluffy.
Add Eggs: Add the eggs one at a time, beating well after each addition.
Add Vanilla and Sour Cream: Reduce mixer speed and mix in the vanilla extract and sour cream until combined. The batter may appear slightly curdled.
Add Dry Ingredients: Gradually add the flour mixture, mixing until smooth and fully incorporated.
Fold in Blueberries: Gently fold the blueberries into the batter.
Layer the Cake: Spoon half of the batter into the prepared bundt pan. Sprinkle half of the streusel evenly over the batter. Add the remaining batter and spread gently to cover the streusel layer.
Bake: Bake for 1 hour 10 minutes to 1 hour 20 minutes until golden and a toothpick inserted comes out clean or with a few moist crumbs. If the tester appears wet from streusel, gently press the top of the cake. A slight spring back indicates doneness.
Cool & Serve: Let the cake rest in the pan for 15 minutes, then invert onto a wire rack to cool completely. Dust with powdered sugar if desired before serving.
Notes
Streusel Butter: Use real butter rather than margarine or spreads to prevent sogginess.
Blueberry Tips: Additional blueberries may be added, ideally sprinkled over the first streusel layer for even distribution.
Doneness Check: Streusel moisture can affect toothpick tests. Use a gentle press test if needed.
Storage: Store in an airtight container at room temperature for up to 1 week. Refrigeration is possible but may dry the cake.
Freezing: Wrap the cooled cake tightly and freeze for up to 3 months. Thaw overnight at room temperature before serving.