I found this on Pinterest and tracked the recipe back to here on Alexandra's Kitchen. I added the topping, just felt it begged for something.
1 cup unsalted butter, room temperature
4 tsp lemon zest or more — zest from 1 large lemon (I used lemon juice)
2 cups sugar
2 eggs, room temperature
2 tsp vanilla
4 cups flour (set aside ½ cup of this to toss with the blueberries)
4 tsp baking powder
2 tsp kosher salt
3 cups fresh blueberries, drained, patted dry
1 cup buttermilk
1 cup butter, soft
1 cup brown sugar
2 tbsp flour
2 tbsp cinnamon
Preheat the oven to 350ºF. Butter a 9x13 pan.
Cream butter with lemon zest and sugar until light and fluffy.
Add the eggs and vanilla and beat until combined. Meanwhile, toss the blueberries with ½ cup of flour, then whisk together the remaining flour, baking powder and salt.
Add the flour mixture to the batter a little at a time, alternating with the buttermilk. Fold in the blueberries. Spread batter into pan.
Mix topping ingredients together and crumble onto the batter. Bake for 35 to 45 minutes. Check with a toothpick for doneness. Let cool at least 15 minutes before serving. If you serve it warm, it's delicious but will fall apart. It's great with milk poured on top of it in a bowl. We waited 2 hours to eat the rest and it had become a fantastic held together cake and very moist.
This serves 6-8 people a good sized slice.