Preheat the oven to 350°F and line a sheet pan with parchment paper. Arrange 24 Ritz crackers on the prepared pan.
Tear 4 slices of white cheddar into small pieces and place evenly over the crackers. Tear the deli ham into bite-sized pieces and layer evenly over the cheese.
Top with the remaining 4 slices of white cheddar, torn into small pieces, then place the remaining 24 Ritz crackers on top to form sandwiches.
In a small bowl, whisk together the melted butter, brown sugar, Worcestershire sauce, and Dijon mustard. Spoon the butter mixture evenly over the tops of the crackers, then sprinkle with poppy seeds.
Bake on the center rack for 8-10 minutes, or until the cheese is melted and the tops are golden brown. Let cool for a couple of minutes before serving.