Place lavender in a tea infuser. In a medium saucepan, bring milk to a simmer over medium-low heat. Turn off the heat, add infuser with lavender, and cover. Steep lavender for 3 to 5 minutes, depending on desired strength of flavor. Remove infuser of lavender. Add the chocolate to the milk and whisk vigorously until chocolate is melted and mixture froths. Pour equal amounts into 4 mugs, top each with a dollop of whipped cream, and sprinkle with chopped pistachios.