Sift together the flour, sugar, baking powder, baking soda, and salt in a large bowl.
Grate the cold butter into the dry ingredients (using a scale makes this very easy) Halfway through, toss the butter into the flour and then continue until you've grated the total amount of butter into the flour mixture.
Toss the ingredients together to coat all the butter with the flour mixture. Work quickly so you don't melt the butter with the heat of your hands.
Add in the chopped bacon and chives and toss to combine
Make a well in the center of your dry ingredients and then add in the cold buttermilk. Pull the dry ingredients towards the center. Make "claw" with your hand or use a fork and mix the dough with the intention of getting rid of all the dry flour. The dough will look rough and shaggy. Don't over-mix it or try to make it smooth.
Turn the dough out onto a lightly floured countertop
Flatten the dough roughly with your hands or rolling pin to about 2" in height. Do your best to lift half the dough and fold it in half. This will be messy! Flatten dough down to 2" again.
Rotate your dough 45º and fold in half again. Repeat 4-5 times. If the dough becomes stretchy or tough, place it in the fridge for 10 minutes to relax the gluten. Then continue folding.
Roll the dough out to 1" thick with a rolling pin dusted with flour.
Use a 3" round biscuit cutter or sharp knife to make your biscuits. Press straight down and lift straight up. Do not twist.
Place the biscuits on a parchment-lined sheet pan with about 1/4" of space in between the biscuits. Brush the biscuits egg wash *see note. Do not brush the sides.
Bake the biscuits at 375° F for 15-20 minutes or until they are golden brown. Rotate the pan halfway through baking for even browning.
Immediately brush the tops of the biscuits with melted butter and sprinkle on flaky salt. Serve warm. Store leftover biscuits in an airtight container in the fridge. Re-heat to refresh them before serving.