• Preheat oven to 375°F. Butter the inside of a 10 inch cast iron pan or a 2 quart baking pan, such as a 9 inch square pan.

Make the biscuit topping

• Whisk together the flour, baking powder and baking soda, brown sugar, and salt. Add in cold butter that has been grated or cut into cubes. Toss to coat with flour mixture.
• Use a finger snapping motion to press the butter and flour together, a process known as "cutting in the butter". You should end up with small and large pieces of butter incorporated into flour with the texture of lumpy sand. 
• Shake your buttermilk well and pour over the butter and flour mixture. Using a danish dough whisk, or sturdy spoon, mix the buttermilk and flour mixture together.

Food processor alternative

• The biscuit topping can be made in a food processor by placing the butter and the ingredients into the bowl of your food processor and pulse 6 to 8 times.
• Pour cold buttermilk through the opening on top and pulse just until the dough comes together. Dump the dough into a bowl and place in the refrigerator while you prepare your fruit.

Mix fruit filling

• Peel peaches if desired. I prefer to leave the skins because I'm lazy. Remove pit from peaches and slice in ¼ inch slices. Add to a large bowl.
• Sprinkle over top of the fruit with brown sugar, cornstarch, vanilla. cinnamon, nutmeg, ginger, and kosher salt. Add lemon juice and toss to combine. Add frozen blackberries and gently fold in.

Assemble the cobbler

• Place fruit in the bottom of the prepared baking dish. Take handfuls of dough and place all over the top of the blackberry peach filling. Just pinch of pieces of dough and dot to cover most of the fruit.
• Brush the top of the biscuit dough lightly with egg wash, then sprinkle with coarse or granulated sugar, if using.
• Bake for 40-50 minutes, or until the biscuit topping is golden brown and peach filling is bubbling around the edges. To test for doneness, stick a toothpick into the biscuit topping and if it comes out clean, it’s done.
• Remove cobbler from the oven, and set the pan on a wire rack. Cool for 10 minutes before serving. Serve warm with sweetened whipped yogurt for breakfast or vanilla ice cream for dessert.