Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan to prevent sticking.

In a large mixing bowl, whisk together the sifted flour, baking powder, baking soda, and salt until well combined. This ensures even distribution of the leavening agents.

In another bowl, cream the softened butter and granulated sugar together using a whisk or electric mixer until light and fluffy, about 3-5 minutes. This incorporates air into the batter, helping the cake rise.

Add the eggs one at a time, mixing well after each addition. Then, mix in the vanilla extract and sour cream until fully combined.

Gradually add the dry ingredients to the wet mixture, stirring gently with a spatula until just combined. Be careful not to overmix, as this can lead to a dense cake.

Gently fold in the blueberries, being cautious not to break them to avoid discoloring the batter.

In a small bowl, combine the brown sugar, flour, and ground cinnamon. Stir in the melted butter until the mixture is crumbly. This will create a sweet, crunchy topping for the cake.

Pour the batter into the prepared baking pan, spreading it evenly. Sprinkle the topping mixture over the batter.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean. The cake should be golden brown on top.

Once baked, remove from the oven and allow to cool in the pan for about 10 minutes before transferring to a cooling rack to cool completely.

Slice the cooled coffee cake into squares and serve warm or at room temperature. Enjoy with a cup of coffee or tea!