Butter one slice of bread and place it on the counter buttered side up. Add shredded corned beef brisket on top. Add two one-inch slices of cheese on top. Spread mustard on the top slice of bread and layer it on, mustard side down. Spread butter on the outer part of the top slice of bread.

Slice the sandwich into a triangle and secure with a toothpick. Place on a baking sheet. Bake the triangles at 400 degrees for 5-7 minutes, until the cheese is melted.