Line the bottom of a 13x9-inch pan with parchment paper (if you allow the paper to come up the short ends, it will be easier to remove the bars before cutting).

In a large saucepan, combine the marshmallows, peanut butter, water, butter, and instant espresso. Set the pan over low heat and cook until the marshmallows melt and the mixture is smooth, stirring frequently with a wooden spoon or spatula. This may take several minutes so do NOT rush this step; we want to create a soft caramel, not one that will become too crisp to eat.

Remove the pan from the heat and fold in the Cheerios. Transfer the mixture to the prepared pan and, using water-moistened fingers, press the mixture into the pan.

Melt the chocolate in the microwave on 50% power, checking and stirring every 30 seconds. Pour the chocolate over the cereal mixture and spread out in an even layer.

Let cool until the chocolate is set (this will depend on the temperature of your kitchen; you can speed things up by refrigerating).

Cut into bars and serve. Store leftovers in an airtight container for up to 3 days or freeze for up to 3 months. No need to defrost before serving.