Step 1Combine the cream and orange zest in a small saucepan and heat just until bubbles form around the perimeter. Remove from the heat and let stand 30 minutes.
Step 2Boil the orange juice until reduced to about 1 1/2 tablespoons and starting to caramelize. Blend into the cream.
Step 3Whisk together the egg yolks and sugar until the sugar dissolves. Reheat the cream to scalding, then pour about one-fourth into the egg yolks, whisking continuously. Whisk the mixture back into the saucepan with the cream. Cook over low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon, about 5 minutes. Stir in the liqueur. Serve warm or chilled.