Ingredients

• 1 1/2 cups sugar
• 1 3/4 cups water
• 2 (14-oz) packages frozen guanabana (soursop) purée such as Goya brand, thawed (3 cups)
• 1 1/2 cups fresh mango purée (from 1 1/2 lb mangoes) or 1 (14-oz) package frozen mango purée, thawed
• 3 tablespoons fresh lime juice, or to taste
• 1/2 teaspoon finely grated fresh lime zest
• Garnish: lime zest
• an ice cream maker

Instructions

1. Bring sugar and 1 1/2 cups water to a boil in a 1 1/2- to 2-quart saucepan, stirring until sugar is dissolved, then reduce heat and simmer, uncovered, until reduced to about 2 cups, 3 to 5 minutes. Transfer syrup to a bowl and cool completely.
2. Whisk together guanabana purée and 1 1/2 cups syrup, then freeze in ice cream maker. Transfer to an airtight container and put in freezer to harden, about 2 hours.
3. Blend mango purée, lime juice, 6 tablespoons of remaining syrup, and remaining 1/4 cup water in a blender until very smooth. Force through a fine-mesh sieve into a bowl, pressing on and then discarding solids, then whisk in zest and, if necessary, additional water for a thin purée. Serve sorbet with coulis.