Put the flour, baking powder, baking soda and salt in the bowl of your food processor or mixing bowl
Pulse a few times to combine, then add the butter and pulse until the texture resembles fine crumbs, or use your hands to combine
Whisk the milk and egg together and pour through the feed tube of your food processor while pulsing until the dough comes together evenly
Turn the dough out onto your counter, lightly floured, and form into a square, the wrap in plastic wrap and refrigerate for at least one hour
Set the oven rack in the top position and heat oven to 425, and line a baking sheet with parchment paper or a Silpat
Remove the biscuit dough from the fridge and roll out into a square about 1/2 inch thick, and cut out shortcakes and place on tray
Brush tops with melted butter and sprinkle with just a bit of coarse salt
Bake until risen and nicely browned, around 5-10 minutes depending upon your oven, and definition of nicely browned