Dice your peaches and lay the pieces in a single layer on a piece of paper towel. Cover with another paper towel and gently press. Let sit while you continue with the next steps. This will pull out excess surface moisture from the berries.
In a large bowl, combine your all-purpose flour, baking powder and baking soda, brown sugar, and kosher salt.
Toss cold unsalted butter cubes into the dry ingredients. Use a finger snapping motion to mix in the cold butter until you have a mixture of flat pieces and crumbly, pea sized bits of butter mixed into the flour.
Add the diced peaches and toss to coat.
Add buttermilk to dry ingredients and use a spatula or dough whisk to gently bring the wet and dry together until you have a fairly uniform, soft dough.
Dump the biscuit dough out onto a lightly floured surface. Using your hands and a bench scraper or flat edge, gently fold the dough over onto itself until it all comes together.
Use a rolling pin or your hands to shape the dough into a rectangle at least ½ inch thick. Using a 2 inch biscuit cutter, or cookie cutter, cut out your biscuits. Gather your scraps as you go, gently re-form them together, and cut as many biscuits as you can. You should get 8 to 12 biscuits. Place on your prepared baking sheet.
Place your biscuits on a parchment paper lined sheet pan. Put the sheet pan of biscuits in the freezer or refrigerator to chill while you preheat the oven to 425°F.
Remove biscuits from the freezer and brush the tops of the biscuits with more buttermilk, or use an egg wash of one egg whisked with one tablespoon of water or milk.
Sprinkle the tops of the biscuits with 1 tablespoon of streusel, if desired. Bake in a 425° oven for 18-20 minutes, or until golden brown.