In the same large bowl, to the self rising flour you just created, add in baking powder, baking soda & salt, and whisk together.
Add the cold cubed or grated butter, and use a fork or pastry cutter to combine the flour, until you have a coarse mixture that resembles peas.
Using a wooden spoon, or a spatula, add in the beaten egg and mix to combine
Add in the buttermilk and continue to stir until the dough comes together into a large mass that is sticky. Cover and let rest in the fridge for at least 30 minutes.
Once the dough has rested, preheat your oven to 400° F / line a baking sheet with parchment paper or a silpat.
Transfer the dough from the bowl onto a lightly floured surface, working quickly so that the butter doesn't melt, use floured fingers to press the dough down, shaping it into a 10"-12" circle, and about 1/2" thick.
Using a floured round 2 3/4" biscuit cutter, cut biscuits out pressing the cutter straight down; avoid turning the cutter as this will seal the edges, preventing the biscuit to rise.
Transfer biscuits onto prepared pan and lightly brush the tops with the egg & buttermilk mixture.
Bake in the middle of preheated oven for 12 minutes, or until the edges and tops develop a beautiful golden brown color.