Preheat the oven to 425 degrees. In a small bowl, mix together the butter and mustard until well blended. Spread the mixture on one side of each slice of bread. For each sandwich, top one slice of bread, butter side up, with a half-slice of cheese, one slice of ham, another half-slice of cheese and a second slice of bread, butter side down. In a medium bowl, beat together the eggs and milk. Dip each sandwich into the mixture. Brown both sides of the sandwich on a lightly buttered griddle. Place on a cookie sheet and bake in oven for 8 to 10 minutes. Slice each sandwich in half, sprinkle with powdered sugar and top with preserves if desired.