• Preheat the oven to 350 degrees Fahrenheit. Spray a 9x13 baking pan with nonstick spray, and set aside.

For the Mixed Berry Filling:

• Add the blueberries, raspberries, blackberries, strawberries, sugar, cornstarch, lemon zest, lemon juice, and vanilla to a large bowl. Gently fold them together to coat the berries in the mixture without smashing them too much.
• Spread the berry mixture into the bottom of the prepared baking pan and set aside.

For the Biscuit Topping:

• Stir the flour, sugars, lemon zest, baking powder, baking soda, and salt together in a large bowl.
• Add the cold butter into the bowl in small cubes.
• Cut the butter into the flour mixture with two forks, or a pastry cutter, until the pieces are smaller than a pea.
• Add 1/3 cup of the buttermilk and stir it into the flour mixture, until just combined.
• Take a few Tablespoons of dough at a time and flatten it out. Lay the pieces on top of the berries, and repeat with remaining dough. Leave some of the berries exposed for a pretty presentation.
• Brush the dough with the remaining 1 TBS of buttermilk with a pastry brush.
• Sprinkle the coarse sugar over the top of the dough.
• Bake the cobbler for 45 to 50 minutes, or until the biscuits are golden brown and the berries are bubbly.
• Let the cobbler cool for at least 10 minutes, then serve.