**Making the cake batter:
Preheat oven to 350F/180C. Line a baking pan 9 x 5 x 3 inch with parchment paper.
In a bowl, sift flour with baking powder and salt. Keep aside.
In a different bowl, beat the sugar and egg until lighter in color, add vanilla, butter, milk, and beat until well combined.
Add the dry ingredients to the wet and gently mix. Add the blueberry and fold gently to avoid the blueberry from bleeding in the batter.
Pour the batter into the previously prepared baking pan. Spread the streusel evenly over the batter, add few blueberries on top and bake for 50 minutes, or until a toothpick inserted in the middle of the cake comes out clean.
**Making the streusel:
In a bowl, mix all ingredients together using a fork to form breadcrumb like texture.