How to Make It
Bring 2 cups sugar and 1 cup water to a simmer in a small saucepan over medium, whisking occasionally, until sugar dissolves, 8 to 10 minutes. Remove from heat, and stir in guavas and lemon zest. Chill, uncovered, until cold, about 1 hour.
Whisk together heavy cream, whole milk, rice, remaining 2 cups water, and remaining 3/4 cup sugar in a large Dutch oven. Using the tip of a small, sharp knife, scrape vanilla bean seeds into Dutch oven; add pods. Bring mixture to a boil over high; reduce heat to low, and maintain a simmer (adjust heat as needed). Cook, stirring occasionally, until rice is tender and mixture is creamy and slightly soupy, about 22 minutes. Remove from heat, and chill, stirring occasionally, until rice mixture has thickened and cooled slightly, about 20 minutes. Remove and discard vanilla bean pods.
To serve, spoon rice mixture into bowls; top each with macerated guavas, and serve with ice cream.