Preheat and Prepare: Preheat your oven to 450°F (230°C). Place the stick of butter in your 9x13 inch baking dish and put it in the oven while it preheats to melt the butter completely.
Mix Dry Ingredients: In a large mixing bowl, whisk together flour, sugar, baking powder, and salt until well combined.
Add Wet Ingredients: Pour the buttermilk into the dry ingredients and stir just until a shaggy dough forms. Don't overmix.
Fold in Ham and Cheese: Gently fold the diced ham, cheddar cheese, and mozzarella cheese into the dough until evenly distributed.
Transfer to Pan: Carefully remove the hot baking dish from the oven. The butter should be completely melted and bubbling. Drop spoonfuls of the dough evenly over the melted butter, creating 12 portions.
Bake: Return to the oven and bake for 20-25 minutes, or until the tops are golden brown and the biscuits have risen significantly.
Cool and Serve: Let cool for 5 minutes before serving. The biscuits will absorb some of the butter as they cool.