Preheat your oven to 400 degrees.
Using your fingers, work the shortening in with the flour in a large bowl until the flour appears crumbly, about the size of little peas. Stir in the buttermilk until almost mixed, adding more if you need it in 1/4 cup increments until it's the right consistency. Mix in 2 cups of shredded cheddar cheese.
Lightly flour a work surface (I use my clean countertop). Place the biscuit dough on the floured surface and press it down with your hands (or use a rolling pin) into a large rectangle. Then cut the large rectange into 2 separate rectangles. (Or divide the dough in half, and make separate dough rectangles) Roll up one of the biscuit rectanges, like you would homemade cinnamon rolls, and place it on a large baking sheet that has been sprayed with nonstick cooking spray. Unroll the biscuit dough on the baking sheet so that it's a rectangle again (this method makes the moving of the dough easier! Check out my video above this recipe to see what I'm talking about as I'm making them in my kitchen).
Spread the pot roast (and any mashed up carrots, onion, and potato from the leftover pot roast) over the biscuit dough rectangle. Sprinkle with the remaining cup (or more!) of shredded cheese. Transport the other biscuit dough rectangle by the roll up method that I describe in the previous step - and unroll it over the pot roast. You should have a biscuit rectangle on the bottom, then pot roast, then the other biscuit rectangle. Pinch the edges of two layers of dough together to seal.
Brush with 1/2 stick of melted butter. Bake at 400 degrees for about 25 minutes, or until biscuits have nicely browned. Brush with the other 1/2 stick of melted butter when they come out of the oven. Slice into squares and serve.