Preheat the oven to 170°C (325°F).
Place the cooked ham in a large roasting tray and wrap it tightly with foil. Bake the wrapped ham for 20–30 minutes, until warmed through.
While the ham warms, melt the butter in a small saucepan over low heat. Stir in the honey, brown sugar, Dijon mustard, and garlic powder until smooth.
Remove the ham from the oven and carefully unwrap the foil.
Brush the glaze generously over the ham.
Return the ham to the oven uncovered and bake for 25–30 minutes, brushing once more with glaze if desired.
Remove from the oven and let rest for 10 minutes before slicing and serving.