Combine the milk, cream, and lavender in a medium saucepan, and bring it to a simmer. Once at a simmer, remove the pan from the heat and cover it with a lid. Let it sit and steep for 30 minutes, to infuse the milk with lavender flavor. (If you want a very subtle lavender flavor, reduce the time to 20 minutes. For a strong flavor, increase to 40 or 45 minutes.)
After 30 minutes, pour the milk through a strainer into a bowl to remove the lavender, then return the milk to the saucepan. Put the pan on the heat and bring it back to a simmer, but don’t let it boil. Remove the pan from the heat and add the chopped milk chocolate, dark chocolate, salt, and vanilla extract.
Whisk everything together until the chocolate is melted and the mixture is smooth. Pour into mugs and top with whipped cream. Lavender Hot Chocolate can be made in advance and stored in the refrigerator for up to a week. Reheat in the microwave or on the stovetop.