Add flour, baking powder, baking soda, kosher salt, and butter in the bowl of a food processor. Pulse until the butter is pea sized. Transfer mixture to a mixing bowl and add in the buttermilk, stirring lightly to combine. Knead until the mixture forms a sticky dough. Add in more flour if very wet and more buttermilk if it is dry. Dust a work surface with flour and pat dough to a 1/2 inch thick disk. Cut out biscuits using a 3-inch round cutter, and reroll out scraps once for additional biscuits. Place the biscuits on a parchment-lined baking sheet, brush with egg wash, and bake in the oven until golden brown.