Line an 8x8 baking pan with parchment or waxed paper, with overhanging sides for easy removal.
In a heatproof bowl set over a bowl of barely simmering water, combine the nut butter, unsweetened chocolate, and coconut oil, stirring until smooth and melted.
Remove from heat and whisk in the sweetener until smooth, then stir in the cereal crisps and stir to coat.
Spread the mixture in the prepared baking pan as evenly as you can. Refrigerate until firm, about 1 hour. Cut into 16 bars.