Preheat oven to 350 degrees F (177 degrees C). Lightly grease a 13x9 baking dish.
Combine all the ingredients for the persimmon filling in a large bowl. Mix and spread it evenly in the baking dish. Bake for 10 minutes. Set aside. Leave the oven on.
In a separate bowl, combine 2 cups of all purpose flour, ½ cup of granulated sugar, 1½ tsp of baking powder, ¼ tsp of baking soda, ¼ tsp salt, and ½ cup of cold, cubed butter. Using a pastry cutter or 2 forks, cut the butter into pea sized pieces. Pour in the buttermilk and mix until combined.
Take handfuls of the dough, flatten it, and place them over the persimmon filling.
Mix the egg and 1 tbsp of buttermilk. Brush the egg wash over the top of the dough.
Mix 2 tbsp of granulated sugar and 1/2 tsp of ground cinnamon. Sprinkle the cinnamon sugar over the top.
Bake for 45-50 minutes until the dough becomes a golden brown. To test of it's done, poke the topping with a toothpick, and if it comes out clean, the dish is done.
Cool for 5 minutes before serving.