How to Make It
Line a large rimmed baking sheet with a silicone baking mat or parchment paper.
In a saucepan, combine the butter, sugar, water and salt and cook over moderate heat, stirring, until the temperature reaches 298° on a candy thermometer, 10 minutes. Remove the pan from the heat and stir in the vanilla and peppermint extracts. Immediately pour the buttercrunch onto the prepared baking sheet and, using an offset spatula, spread it in an even layer; let cool completely.
In a medium glass bowl, heat half of the chocolate in a microwave oven at high power in 30-second intervals, stirring between bursts, until just starting to melt. Stir in the remaining chopped chocolate until smooth.
Spread half of the melted chocolate on the cooled buttercrunch. Sprinkle half of the peppermint on top; let stand until set.
Gently set another large baking sheet on top of the buttercrunch and carefully invert the baking sheets; peel off the baking mat or parchment. Spread the remaining melted chocolate on the buttercrunch and sprinkle with the remaining peppermint; let stand until the chocolate is completely set. Break the buttercrunch into pieces before serving, storing or gifting.