Split a vanilla bean, scrape out the seeds, and save them for another recipe or for flavoring the marshmallows. If you have an empty vanilla bean pod hanging around from another project, this is a great way to use it up. You by no means need a freshly split bean.
In a roughly 2 quart pot, combine the milk and cream with the empty vanilla pod. Bring to a simmer, then shut off the heat and steep for roughly one hour (or up to overnight, covered and refrigerated).
Meanwhile, preheat the oven to 300°. If using a big chunk of white chocolate, give it a rough chop first. Thin bars or discs can be used as-is. Put the white chocolate in a glass or ceramic baking dish, toss it in the oven and “roast,” stirring every 10 minutes or so.
It may seem lumpy and crumbly as you stir, but that’s okay. Keep going until it takes on toasted almond color, about 30 minutes total.
Return the vanilla-milk to a simmer, add the caramelized white chocolate and whisk until smooth. Season with a pinch of salt, ladle into mugs and enjoy.