Line an 8×8-inch baking pan with parchment paper so that two of the sides extend over the edges of the pan for easy removal. Lightly spray the two sides of the pan that are not covered by the parchment paper.

If the nut butter is closer to a solid than a liquid, warm it in a small saucepan with the honey over low heat, stirring often, until thin enough to easily mix with the cereal.

In a large bowl, combine the Cheerios, nut butter, and honey, mixing with a large spoon or your hands until the cereal is well coated.

Transfer to the prepared pan and press firmly in an even layer. Cover with foil and freeze for two hours. Cut into nine squares.

Optional: Melt the chocolate in a double boiler (a heatproof bowl set over a saucepan of simmering water). Drizzle over the bars, then return to the freezer for a few minutes, until the chocolate hardens.