Place stick of unsalted butter on counter and let warm up to room temperature.
In a mixer with the paddle attachment, whip the butter until soft, smooth, and fluffy.
Add confectioner's sugar little by little until fully incorporated.
Add milk, making sure buttercream is smooth. (Add more milk, 1 tbsp at a time, if buttercream is too thick.)
Add in vanilla extract.
Toast multi-grain bread.
Separate buttercream equally into separate bowls.
Add 1-2 drops of food coloring per bowl, and mix by hand until combined.
Take one slice of toast, and add 1 tbsp. each of colored buttercream onto toast.
Using an offset spatula, gently press down and spread buttercream into desired design.
Top with sprinkles or edible glitter, if desired, and serve.