Combine the dried culinary lavender with the dairy-free milk and refrigerate to infuse for at least 2 hours or overnight. Strain out the lavender before use, discarding the solids and keeping the infused milk. Using a hand mixer or stand mixer fitted with the paddle attachment, beat the dairy-free butter and vanilla extract for 1 to 2 minutes until creamy. Gradually add 2 cups of icing sugar and beat on medium speed until incorporated. Add another cup of icing sugar and continue mixing. As the mixture thickens, slowly add the lavender-infused dairy-free milk a little at a time until you achieve a thick, creamy texture. If the frosting is too thin or tastes overly buttery, add an additional ½ to 1 cup of icing sugar; if it’s too thick, add a bit more dairy-free milk. Finish by adding a pinch of salt to taste and a few drops of gel food coloring if desired, then beat until the frosting is smooth and creamy. Store the frosting covered in the fridge or pipe it directly onto cupcakes or cake.