Preheat oven to 400°.
I baked my biscuits in a 10" cast iron skillet you can bake them on a baking sheet as well.
Place the pancake mix in a medium bowl with the baking powder and mix together with a fork.
Add the vinegar to the milk and let it "curdle" a bit, about 1 minute.
Add the milk mixture, butter, and salt to the pancake mix, and stir everything with a fork to combine.
Place the dough on a sheet of wax paper.
Shape it into a disk shape.
Cut it into 4 even pieces.
Gently shape each portion to a roundish shape, don't roll it or press firmly, just gently push the corners in.
Place the dough on the skillet or parchment-lined baking sheet.
Bake for 15-18 minutes or until a toothpick comes out dry.
Remove biscuits from the oven when done, and brush the tops immediately with butter.