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The heat will help solidify the foam. Do not microwave the jar with the lid on, or you may ruin your microwave. Add a sprinkle of some cinnamon or cocoa powder on top of the foam. For something sweeter and fancier, add a drizzle of caramel or chocolate syrup instead.
Open the jar, and microwave it for 30 seconds. Add some garnish, if desired, and serve.