Preheat oven to 350 degrees F. Spray a 9-inch cake pan with nonstick spray. Set aside.
Add butter and sugar to the bowl of a stand mixer. Cream until smooth. Add egg, yogurt and vanilla extract and mix. Add salt, baking powder and 1 cup flour to the bowl and mix until just combined. Add half the milk and mix again. Add remaining cup of flour and mix until just combined. Pour in last of the milk and mix until it just comes together. Stir in blueberries by hand.
Pour batter into prepared pan and set aside.
For the streusel: add all ingredients into a bowl. Using a pastry cutter or two forks, cut together the ingredients until it resembles coarse sand. You can use your hands as well. Sprinkle the streusel evenly over the cake batter.
Bake for 35 to 40 minutes until a toothpick comes out clean.