Creating this crumbly blueberry coffee cake is a straightforward process. Follow these steps for perfect results:

Step 1: Preheat and Prepare

1. Preheat your oven to 350°F (175°C).
2. Grease and flour a 9×13-inch baking dish or line it with parchment paper.

Step 2: Make the Cake Batter

3. In a large mixing bowl, cream together the sugar and butter until light and fluffy.
4. Beat in the eggs, one at a time, then stir in the vanilla extract.
5. In a separate bowl, combine the flour, baking powder, and salt.
6. Gradually add the dry ingredients to the creamed mixture alternately with milk, beginning and ending with the flour mixture. Mix until just combined.
7. Gently fold in the blueberries to distribute evenly throughout the batter.

Step 3: Prepare the Streusel Topping

8. In a small bowl, mix together the flour, brown sugar, and cinnamon for the streusel topping.
9. Cut in the cold butter using a pastry blender or fork until the mixture resembles coarse crumbs.

Step 4: Assemble and Bake

10. Pour the cake batter into the prepared baking dish, spreading it evenly.
11. Sprinkle the streusel topping evenly over the cake batter.
12. Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
13. Allow the cake to cool in the pan on a wire rack before serving.