Preheat oven to 350°. With a hand mixer or in a stand mixer, cream together butter, sugar, and lemon zest.
Add the egg and vanilla; mix till combined. In a separate bowl, 3 1/2 c. whisk together flour, salt, and baking powder.
Place blueberries in a small bowl and toss with 1/2 c. flour. Add the flour mixture a little at a time to the sugar mixture, alternating with the buttermilk. Carefully fold in blueberries.
Prepare topping by placing flour and both sugars in a small bowl. Cut up the butter into small pieces and mix with sugar with fingers until it turns into a crumble.
Place batter in a greased 9x13 pan. Sprinkle with topping and bake for 35-40 minutes, or until a butter knife inserted comes out clean.