In a medium metal bowl, combine white chocolate, coconut oil, and lavender. Set over a saucepan with 1 in. simmering water over low heat. Heat, stirring occasionally, until chocolate melts. Add lemon zest and food coloring, if using, and stir to combine.
Whirl chocolate mixture in a blender until very smooth, about 30 seconds. Strain through a fine-mesh strainer into a bowl or jar. Let cool. Use at room temperature (if magic shell happens to firm up, just microwave to about 105°). Stir, then spoon over ice cream.
Make ahead: At room temperature, airtight up to 1 month. (If topping is chilled, it firms up; if needed, microwave just until pourable.)