In a small bowl, mix the mustard and beer. Set aside.
In a microwaveable dish, melt the butter for 30 seconds. Set aside.
Place each slice of the marble rye on a flat surface. Spread each slice with some of the beer mustard. On the bottom bread of each sandwich, portion 2 slices of Cooper Sharp cheese. Add a layer of corned beef. Top the corned beef with a second layer of 2 slices of Cooper Sharp cheese, followed by another layer of corned beef. Top with equal portions of cooked cabbage and a few more Cooper Cheese slices. Press the top with remaining bread slices, beer mustard side down. Using a brush, coat the outside top of the sandwiches with melted butter.
Place a medium skillet over medium heat or set your griddle to 350°F.
Place butter side of sandwich on hot surface and grill for 3-4 minutes. While the first side of sandwich is grilling, brush the other outside slice of bread, now on top, with butter.
Flip and continue to grill for another 3-4 minutes until bread is toasted and cheese is melted. Remove from heat.
Use any remaining beer mustard as a dipping sauce for the sandwiches or with a side of fries.