I would use a circle cutter. Cut it out of fondant mixed with a little tylose. Or use 50/50 fondant/gumpaste. If you want to make them bigger than your cutter, find something with a lid the size you want and trace around it with an exacto or wheel cutter. Then use the same cutter to cut a small moon shape at the bottom of the ears to form on the hat. Depending on the size, you may need to stick a couple toothpics or skewers into the bottom to stick onto the hat. Let them dry for a day or so to firm up. Use some gum glue to glue the ears to the hat.
This is how I did ears for my mickey mouse. It was an actual mickey mouse head but I would think you'd do the same thing for a hat.