Twenty minutes before starting, place the butter in the freezer.
In a large bowl, combine the flour, salt, sugar, and baking powder. Grate partially frozen butter over the flour mix, using a cheese grater.
Mix well using a fork.
Pour in the milk and mix the ingredients together using a spatula.
Add the mix-ins, if any, and mix together.
Turn the dough onto a generously floured work surface, dust the top with flour, and press into a rectangle about 3/4” thick, incorporating any loose pieces. Use a metal scraper to lift the dough and add more flour underneath. Now, using a rolling pin, roll the dough out into a 1/2" thick rectangle. Fold like you would an envelope, dust with more flour, and roll out to ½” thick. Rotate 90 degrees and repeat folding and rolling out. Then rotate 90 degrees and repeat folding and rolling out two more times, making it a total of 4 times folding and rolling out.
Cut out biscuits using a 3” round cookie cutter. Gather the cut-offs, roll them out again and cut more biscuits. Repeat until all dough is used. Place the biscuits on a baking sheet lined with parchment paper, spacing out at least ½’, and let rest for 20 to 30 minutes. Ideally, place them in a refrigerator or a very cool room.
Twenty minutes before baking, preheat to 500F.
Whisk the egg yolk with one teaspoon of milk in a small bowl. Brush the tops of biscuits with the egg wash.
Transfer the biscuits to the oven, lower the temperature to 450F, or 425F if using convection (preferred).
Bake for 8 minutes, rotate the baking pan and bake for additional 6-8 minutes, or until the biscuits have risen about 1.5 times, and the tops and the bottoms of the biscuits are rich golden brown.
Remove the biscuits from the oven, transfer to a cooling rack and let cool for 3 minutes before serving.