Keep leftover strawberry syrup in a sealed jar in the fridge for up to one week, or freeze it in an ice cube tray for single servings. Frozen syrup cubes double as the base and the ice. Oat milk gives the cleanest white middle layer; almond milk blurs the layers faster. Use ceremonial-grade matcha and sift it before whisking to avoid clumps. The syrup needs to coat the back of a spoon — if it is too thin, the layers will turn pink. Serve the drink unstirred so the stripes hold.