In a medium saucepan, combine cream, orange zest, lemon zest, granulated sugar, and vanilla sugar.
Stir in orange juice and grapefruit juice until smooth.
Heat the mixture on low until it starts to bubble, then remove from heat.
Pour into grapefruit peel halves for serving; fill extra cups with remaining mixture.
Refrigerate for 2-3 hours until set.
Garnish to your taste (optional). Enjoy this silky, citrusy treat!