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

In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy, about 3-4 minutes. This incorporates air into the mixture, which helps the cake rise.

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

In a separate bowl, whisk together the sifted flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing on low speed just until combined. Be careful not to overmix, as this can lead to a dense cake.

Gently fold in the blueberries using a rubber spatula, being careful not to break them.

In a medium bowl, combine the flour, brown sugar, and ground cinnamon. Pour in the melted butter and mix with a fork until crumbly. This will create a delicious topping that adds texture and flavor.

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

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean. This ensures the cake is fully cooked and not gooey in the middle.

Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes. Then, transfer it to a cooling rack to cool completely.