With an electric mixer, beat together the yolks and sugar for 4 to 5 minutes until the color slightly changes and the mixture becomes very well combined and thickens. Then beat in the mascarpone until smooth. Then whisk in half of the espresso and rum until just combined. You don’t necessarily need to measure so you are adding exactly half of each, but if you want to, that’s fine. The main point is that you will want to reserve some liquid for mixing with the cereal.

In a large mixing bowl, drizzle the remaining espresso and rum over the cinnamon toast crunch. Use a large wooden spoon to quickly stir everything around so all the cereal gets coated a little but nothing is soggy.

Assemble the tiramisu! In four little cups or wine glasses (whatever you have really is fine), add a layer of cereal, then the custard mix, then more cereal, and then another layer of custard. Cover in plastic wrap and refrigerate for at least 2 hours, but up to 8 hours.

When you are ready to serve, dust the tops with cinnamon. These are best served within the 2 to 8 hour range after they are assembled. If you want to keep them longer than that, I would hold off on mixing the cereal with the liquids and assembling until you are getting closer to serve time. One of the batches I made I kept one for about 12 hours to see how it tasted, and I thought it was a little too soggy by then.