Pour orange juice into a medium sauce pan and place over medium heat. Stir in sugar, mix and once it comes to a gentle simmer about 4 to 5 minutes later, let it cook for 5 minutes. Let cool. In another sauce pan, pour milk and place over low medium heat, once it comes to a simmer, about 4 to 5 minutes later, remove from the heat. In a medium bowl, beat the egg yolks with the heavy cream, cornstarch and salt until thoroughly combined. Gently, pour the milk mixture in a thin stream into the egg yolks, beating well with a whisk or fork. Once mixed, pour the grapefruit mix into that egg yolk mix in a thin stream whisking as you do. Return entire mix to a sauce pan and place over low medium heat, let it come to a gentle simmer and cook until it coats the back of a wooden spoon, stirring occasionally, about 15 minutes. Remove from the heat. Mix in the completely mashed avocado, whisking thoroughly with a whisk or fork until thoroughly combined. Strain the mixture, using a fine strainer, onto a bowl. Chill the mixture by placing the bowl over an ice bath. Cover the natilla with plastic wrap placed directly onto the custard so it will not create a film. Place in the refrigerator and let completely chill. Serve by spooning the natilla into bowls and garnish with cacao nibs.