Combine the butter, maple sugar, mustard, cinnamon, and ginger in a saucepan over medium heat. Bring to a bubble while stirring, reduce the heat to low, and allow to reduce until it coats the back of a spoon.
If the ham still has a rind, remove this layer with a sharp knife. Score the surface of the ham in a diamond pattern.
Place the ham in the center of a cast iron skillet and baste with a thin coat of the glaze. Bake for 30 minutes.
Remove the ham from the oven and turn the heat up to 400°F. Baste again, this time with a bit more glaze, and place in the oven to bake for 15 minutes. Continue to baste the ham every 15 minutes with the glaze, as well as drippings from the skillet, until the internal temperature of the ham reads 145°F. Slice the ham very thinly for serving.