Combine sugar and egg yolks in a saucepan. Gradually stir in black-currant juice. Cook over low heat, stirring constantly, until mixture coats the back of a spoon. Do not allow mixture to boil, or it will curdle.
Remove from heat and whisk mixture until slightly cooled. Stir in butter, 1 tablespoon at a time, and add cassis. Refrigerate until ready to use.