Preheat the oven to 425°F (218°C). Grease or line a baking sheet with parchment paper.
Stir All-Purpose flour, Cake flour, baking power, baking soda, sugar, salt, granulated garlic, and old bay together in a bowl. Mix until well combined.
Grate the butter using a box grater or cut into small cubes. Gently crumb the butter into the flour mixture. When finished, the butter and the dry ingredient mixture should resemble a coarse meal.
Mix the sour cream, mashed potatoes (follow this mashed potato recipe if you’d like to make them from scratch), cheddar, and green onions together in a separate bowl.
Gently combine the wet and dry ingredients. Do not overmix.
Turn the dough out onto a lightly floured work surface. Dust the top of the dough with flour and pat it down with your hands until it is about 1-inch thick.
Fold the dough gently onto itself like an envelope, then rotate the dough 90 degrees and fold it like an envelope again. Rotate the dough 90 degrees again and repeat the fold one more time, pressing or gently rolling the dough about 1 inch thick each time.
Cover the biscuit dough with plastic wrap and allow it to rest for about 15 minutes in the fridge. In the meantime, clean up your work surface and apply a light dusting of fresh flour.
Remove the dough from the fridge and place it on the floured surface. Gently press or roll the dough to a one-inch thick rectangle about 6 inches by 12 inches.
Using a very sharp knife cut the biscuits into 3×3 inch squares. (You could also use a biscuit cutter for round biscuits but cutting the biscuits into squares ensures no wasted dough. If using a round cutter, you can rework the scraps to make more biscuits; however, working the dough more means the second batch of biscuits will be slightly tougher than the first.)
Place the biscuits on a non-stick baking sheet or one lined with parchment paper. Brush the biscuits with the beaten egg and water mixture. Bake the biscuits for about 15 minutes.
Mix the garlic butter together.
Take the biscuits out when they are golden brown.
Brush with the garlic butter while hot.
Serve and enjoy!