In a large saucepan, whisk together eggs, agave, and salt. Whisk in 1 cup of the milk/cream. Cook over low heat, stirring constantly, until the mixture leaves a thin film when tested on the back of a metal spoon. Remove from heat. Stir in the remaining cup of milk/cream and vanilla.

Cover and refrigerate several hours or overnight—until it is thoroughly chilled. Pour into cups and garnish with desired garnishes. If you wish you can add more brandy or rum to the chilled eggnog.