Depending on the freshness of your ground cardamom powder you may need more or less. If you are using fresh or home ground cardamom, you will only need ½ teaspoon for perfect flavor and aromas. If it's older you may need up to 1 teaspoon.
You can also melt white chocolate with a double boiler or bowl set over a simmering pot of water. Make sure to keep the water at just a simmer, not a rapid boil. Make sure the bowl of chocolate does not touch the water when it is set in place in the pot. And keep water from splashing into the chocolate. Go slow, stir, and don’t overheat the chocolate.