Trim, wash, and pat rhubarb dry. Dice into 1/2-inch cubes. Add sugar to water, bring to a boil, then add rhubarb. Return to a boil, then simmer for about 10 minutes. Add vanilla, salt, and pepper. Taste for sweetness and add more sugar if desired. Cool, then refrigerate. When thoroughly chilled, garnish with an egg-size scoop of mascarpone or sour cream.