Slice the Hawaiian rolls horizontally in half, keeping them connected as a whole.
Set aside the top portion and place the bottom layer into a lightly buttered 9x12 baking pan.
Layer each roll base evenly with one slice of ham and half a slice of Havarti cheese.
In a microwave-safe bowl, melt the butter and mix in Dijon mustard, Worcestershire sauce, onion powder, and poppy seeds until fully combined.
Replace the top half of the rolls and use a silicone brush to generously coat the tops with the buttery poppy seed glaze.
Cover the baking pan loosely with foil and bake at 350°F for 15 minutes.
Take off the foil and bake for an additional 5 minutes until the cheese is bubbly and the tops turn golden brown.
If the buns brown too fast before the cheese melts, cover again with foil and continue baking until done.