Bake 11–13 minutes, until the edges are just lightly golden. Cool completely.
Whisk powdered sugar, milk, vanilla, and salt until smooth and pourable. Spoon or drizzle icing over each cooled cookie.
Immediately sprinkle with sanding sugar. There should be 1/3 of each sugar flavor: Queen Charlotte Vanilla, Lady Whimbelton Lemon, Penelope Sweet Strawberry