Preheat your oven to 350 degrees F. Set out a baking sheet and cover it with parchment paper or aluminum foil.
Slice the buns in half carefully so the tops and bottoms aren’t crushed in the process. Set up all 8 or 12 buns on the pan, pushed together so the sandwich can be made as one large unit.
Spread the mayonnaise and mustard on the top buns. Lay out the corned beef on the bottom buns, splitting the meat equally between all of them. Layer the cheese on.
Put the tops of the buns on and brush them with the melted butter. Cover the sandwiches with a loose tent of aluminum foil, then place them in the oven. Bake for 15 minutes and separate the sliders with a knife before serving.