Preheat the broiler. Butter the bread slices on both sides. Top 4 slices of bread with 8 thin slices of Gruyère, followed by all 8 slices of ham. Finish with the remaining Gruyère on top of the ham, and then lay the remaining 4 bread slices on top of the cheese.

Put the croques monsieur onto a baking sheet, and place the baking sheet under the broiler for 5 to 8 minutes, or until the bread turns golden brown and crispy. Flip the sandwiches over carefully, and brown the other side. Remove the four sandwiches from the broiler, trim off the crusts, and cut each sandwich into eight pieces. Serve immediately..

If you plan to make the croques monsieur ahead of time, they can be laid between layers of parchment paper and refrigerated