In a large saucepan heat half-and-half until steaming.
In a medium bowl whisk to combine yolks, agave, molasses, ginger, nutmeg, cinnamon, and salt. Pour half of hot half-and-half into yolk mixture, whisking constantly. Pour yolk mixture into remaining half-and-half.
Gently heat half-and-half mixture until it reaches 180℉. Take off heat, let cool completely at room temperature, then whisk in rum and vanilla.
Refrigerate until chilled, 8 hours. Eggnog will keep, refrigerated, up to 1 month.