Preheat the oven to 375 degrees.
Grease an 8x8 baking tin with shortening or butter.
To make the streusel- in a medium bowl combine the sugar, flour, cinnamon, and salt. Mix together until combined. Add the butter and using your fingers, mix together until evenly distributed and crumbly. Set aside.
In a separate medium mixing bowl add the flour, baking powder, and salt. Whisk to combine.
In a larger mixing bowl beat together the butter and sugar until fluffy. Add the egg and the vanilla. Beat until well combined.
Add the flour mixture in 3 additions, alternating with the milk. Beat until just combined, but don't over-mix. The batter will be quite thick.
Fold in the blueberries carefully.
Spread into the prepared pan and sprinkle with the struesel topping.
Bake for 40-45 minutes or until toothpick inserted into the center comes out clean.
Cool in the pan on a cooling rack before slicing into squares.