Preheat oven to 350°F (175°C). In a small bowl, combine flour, brown sugar, cinnamon, and salt. Stir in softened butter and vanilla until a soft, crumbly dough forms. Put it in the fridge.

In a large separate bowl, cream butter and sugar until fluffy (2–3 minutes). Add eggs one at a time. Stir in vanilla, lemon zest, lemon juice, and sour cream. Make sure to zest lemon before cutting it in half!‍

In a separate bowl, whisk flour, baking powder, baking soda, and salt. Gently fold dry ingredients into wet until almost combined.

In a separate bowl toss blueberries and flour till combined and then pour it into the cake batter and gently combine.

Spread batter into a greased or parchment-lined 8x8-inch pan. Sprinkle a few blueberries on top if there are any leftover. Add the crumble streusel evenly over the top.

Bake at 350°F (175°C) for 45-48 minutes, or until a toothpick inserted in the center comes out with moist crumbs. Let cool in pan for 15 minutes. Remove cake from can and place on a cooling rack till cool. Finish with powdered sugar.