Whisk together the flour, psyllium, baking powder and salt in a large bowl and put the bowl in the freezer for 15 minutes. Put the butter cubes on a saucer and put them into the freezer as well.
Put the flour mixture into the bowl of a food processor and whirl them up to aerate the flour and combine the ingredients. Add the butter cubes and pulse the food processor until the butter has been broken down into pieces the size of lima beans. Pour in the buttermilk and yogurt and pulse until everything is just combined.
Put the biscuit dough onto a piece of parchment paper, being careful to remove the food processor blade. Using the heel of your hand, press the biscuit dough together very gently. Form the dough into a disc 1 inch in height.
Grease a 9-inch skillet. Using a biscuit cutter or a small glass, cut straight down into the dough and cut out as many biscuits as possible. (Do not twist the biscuit cutter, as that will blunt the edges of the biscuit dough and prevent them from rising.) Place the biscuits into the prepared skillet, nestling them right up against each other, which will nudge the biscuits to rise taller. Gather all the scraps of dough into another tall disc and cut as many more biscuits as possible. Put the skillet into the freezer for at least 1 hour.
Preheat the oven to 425 degrees F. Take the skillet from the freezer and put it directly into the oven. Bake until the biscuits are firm and light golden brown, 12 to 14 minutes. Brush the tops of the biscuits with the melted butter and let them rest in the pan for 10 minutes, then serve immediately.