Line a large baking sheet with parchment paper. Preheat oven to 450°F.
In a mixing bowl, whisk together all-purpose flour, baking powder, baking soda, sugar, and salt.
Toss cold butter cubes into the dry ingredients and, using a pastry cutter or your fingers, squeeze, pinch, and mix in the cold butter until you have a mixture of flat pieces and crumbly pea sized bits of butter mixed into the flour.
Food processor instructions: Place cubed butter in the freezer for 10-15 minutes while you get all your other ingredients together. Measure all of your dry ingredients into the bowl of your food processor. Pulse a few times to mix.Add chilled butter to the dry ingredients and pulse 5 to 6 times, or until most of the butter is in pea sized, or smaller, pieces. Pour into a mixing bowl and proceed with the rest of the instructions. Add Gruyère cheese, chopped bacon, and chopped chives to your flour/butter mixture and toss to distribute evenly.
Make a well in the center of the mixture and add the cold buttermilk. Using a danish dough whisk, or a large silicone spatula, mix the wet and dry ingredients together until you have a fairly uniform, loose dough. The flour won't look all mixed in and that's okay. It will come together.
Dump the butter/dry ingredient mix out onto a lightly floured surface. Using your hands, gently press everything together into a rough square.
With a bench scraper or knife, cut into four smaller squares. Stack the pieces of biscuit dough on top of each other. Smush it all down into a square again. Repeat this cutting and stacking procedure 2 more times, for a total of 3. Pat or roll the dough into a rectangle at least ½ inch thick.
Using your bench scraper or a knife, cut into square biscuits and place on a parchment paper lined sheet pan. Place in the freezer for 10-15 minutes before baking.
Remove biscuits from the freezer and brush the tops with an egg wash made of one egg whisked with one tablespoon of water. Sprinkle with flaky salt, if desired.
Bake in a 450° oven for 15-18 minutes, or until golden brown. Eat warm.