1. Preheat the oven to 375ºF (190ºC).
2. To make the blueberry filling, mix the blueberries in a 10-inch (23cm) skillet or baking dish with the granulated and brown sugars, cornstarch, lemon zest and juice, and gin. Place the pan on a foil or parchment-lined baking sheet (to catch any spills), and bake until the berries start bubbling, stirring once midway during baking, about 30 to 35 minutes.
3. In a measuring cup, mix the milk with the vinegar and let stand until curdled, about 10 minutes.
4. To make the biscuits, whisk together the flour, sugar, baking powder, baking soda, and salt in a medium bowl. Add the butter and use a pastry blender, or your hands, to break the butter into pieces the size of corn kernels. Give the milk a brisk stir with a fork to bring it back together, then add it to the dry ingredients.
5. Stir the mixture gently with your hands or a spatula until it comes together, but avoid kneading it; overworking the batter will make the biscuits tough. It should be somewhat shaggy, not smooth. If the mixture seems too dry, add another tablespoon of milk.
6. Drop the biscuit batter over the warm cooked blueberries in six dollops, evenly spaced apart. If you want smaller biscuits, you can drop it in eight dollops.
7. In a small bowl, make the egg wash by stirring together the egg yolk and teaspoon of milk together with a fork. Dab the egg wash over the biscuits with a brush. Sprinkle the 2 teaspoons of turbinado sugar over the biscuits.
7. Bake the biscuit-topped cobbler until the tops of the biscuits are golden brown and the biscuits are cooked through, about 20 to 25 minutes.