1. Preheat the oven to 350°F. Spray a 9×13 baking dish with cooking spray, set aside.
2. In a large bowl add the blueberries, raspberries, blackberries, sliced strawberries, sugar, cornstarch, lemon zest, lemon juice, and vanilla extract. Gently stir to combine so all the berries are coated in the mixture. Try not to damage the berries too much.
3. Spread the berry mixture into the prepared baking dish.
4. In a large bowl stir together the flour, sugar, brown sugar, lemon zest, baking powder, baking soda, and salt. Make sure to break up any clumps from the brown sugar.
5. Add the cold butter, either diced or grated, and toss with the flour mixture.
6. Using a pastry cutter or fork, cut the butter into the mixture until the butter is in small pieces no bigger than a pea.
7. Add 1/3 cup of the buttermilk and stir until combined; this will be a slightly sticky wet dough.
8. Take a few tablespoons of the dough and flatten it to about 1/4 inch thick. Lay the pieces on top of the berries, leaving some berries exposed.
9. Brush the dough topping with the remaining buttermilk using a pastry brush. Sprinkle the decorators sugar on top.
10. Bake for 45-50 minutes until the topping is golden brown and bubbly along the edges.
11. If unsure if it is done, poke the topping with a toothpick; if it comes out clean, it’s ready.
12. Let cool for at least 10 minutes before serving.