Line an 8x8-inch baking sheet with parchment paper, leaving a 2-inch overhang on two sides (which will make it easy to lift the bars out when it's time to slice them).
Add the peanut butter and honey to a large heat-safe bowl. Warm in the microwave for 15-30 seconds, or until easy to stir. (This may not be necessary if you keep your peanut butter and honey at room temperature.) Stir together to combine.
Add the cereal and vanilla, if using, to the bowl. Stir gently, but thoroughly, using a wooden spoon or spatula. It may take a minute or two to get the mixture evenly coated.
Press mixture into the prepared baking pan, gently pressing evenly and into the corners.
Chill for at least 20 minutes. Remove from the fridge, lift out using the parchment paper, and slice on a cutting board.
Wrap each bar in plastic wrap (optional) and store in an airtight container in the fridge. Serve cold.
Notes
I like to wrap each bar, then store in a storage bag or airtight container in the fridge. You can skip the plastic wrap, but do store them in an airtight container.
Store in the fridge for up to two weeks. (Ones made with Cheerios are likely going to be fine in the fridge if tightly wrapped for up to a month. Other brands may start to taste stale after 1-2 weeks.) You can also wrap and store in the freezer for up to 3 months.
Nut-free: Use sunflower seed butter.
You can add a little vanilla extract for additional flavor.
These hold their shape best when stored in the fridge and served cold. They soften a little at room temperature.
Brown rice syrup is an alternative to honey that is a little less sweet tasting and stickier, so it helps the bars hold together a little better. You should still plan to serve them cold for best results.