Step 1: Prepare the Oven & Pan

1. Preheat oven to 350°F (175°C).
2. Grease and flour a 9-inch bundt pan or 9×9 baking dish.

Step 2: Make the Streusel Topping

1. In a small bowl, mix together:

   • Brown sugar
   • Flour
   • Cinnamon

2. Add melted butter and stir until crumbly. Set aside.

Step 3: Make the Cake Batter

1. In a medium bowl, whisk together:

   • Flour, baking powder, baking soda, and salt

2. In a large mixing bowl, beat together:

   • Butter and sugar until light and fluffy (about 2 minutes).

3. Add eggs, one at a time, mixing well after each.
4. Mix in sour cream and vanilla extract.
5. Gradually fold in the dry ingredients, mixing until just combined.

Step 4: Assemble the Cake

1. Pour half of the batter into the prepared pan.
2. Sprinkle half of the streusel topping over the batter.
3. Gently fold blueberries into the remaining batter and spread it over the first layer.
4. Top with the remaining streusel topping.

Step 5: Bake & Cool

1. Bake for 40-50 minutes (for a bundt pan) or 30-40 minutes (for a square pan), until a toothpick inserted in the center comes out clean.
2. Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Step 6: Serve & Enjoy!

1. Dust with powdered sugar for a beautiful finish.
2. Serve warm or at room temperature with coffee or tea.