Use blue food coloring to tint vanilla frosting. Reserve a few tablespoons untinted in a ziplock bag; snip off a corner when you’re ready to make the castle. For each cupcake, first use a black food writer to decorate marshmallows with doors and windows. Working with one cupcake at a time, add a generous layer of blue frosting. Assemble the castle by joining the marshmallows and white chocolate chips with dots of the untinted frosting.