Add cream, milk, sugar, lavender and salt to a saucepan over medium heat. Stir constantly until the sugar dissolves. Continue to heat until the mixture becomes warm and begins to bubble around the edges. Remove from heat, add in vanilla extract and allow the mixture to sit at room temperature for 60 minutes while the lavender seeps in. After an hour, strain the milk through a fine mesh sieve and discard the lavender. Add the mixture to a bowl and refrigerate until cold, at least 30 to 60 minutes.