To clean, dry bowl of stand mixer, add chilled, reduced aquafaba, cream of tartar, and xanthan gum and beat on high until foamy. Continue to whisk, adding sugar, 1 spoonful at a time, waiting until each spoonful of sugar is fully incorporated before adding the next. Whisk until mixture is glossy and stiff peaks form. Turn off mixer, scrape meringue from beater into bowl. Add blue matcha, if using, and gently swirl into meringue batter. Don’t worry about mixing it in fully—you are looking for a subtle, marbled effect.
