Preheat oven to 375°F. Line an 8x8" pan with parchment paper.

Start by making the streusel. In a medium mixing bowl, whisk together the flour, sugars, and salt. Add the cold butter. Use your fingertips to pinch the butter into the dough until well distributed and mixture is sand-like. Store in fridge while preparing the cake.

In a medium mixing bowl, whisk together flour, baking powder, and salt. In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, combine the butter, sugar, and lemon zest. Beat on medium speed until light and fluffy, about 2 to 3 minutes. Slowly beat in the milk, yogurt, lemon juice, and vanilla.

Toss 2 cups blueberries in 1 tbsp flour. With the mixer on low speed, add a third of the flour mixture to the butter mixture at a time. Mix until a smooth batter forms. Fold in flour-tossed blueberries.

Spread the batter into the prepared baking pan. Sprinkle with chilled streusel topping and the remaining ⅓ cup blueberries.

Bake for 50-55 minutes or until top is golden brown and toothpick inserted comes out clean or with a few moist crumbs. Let cool for at least 30 minutes before slicing and serving. Enjoy!