In a food processor, blend pistachios until finely chopped. Add honey (or condensed milk), cream, and butter. Blend until smooth and thick. Set aside.
Melt the dark chocolate in a heatproof bowl over simmering water (or in the microwave in short bursts), stirring until glossy and smooth.
Spoon a thin layer of melted chocolate into a silicone chocolate bar mold, tilting to coat the bottom and sides. Chill briefly to set.
Spread the pistachio filling evenly over the set chocolate, leaving a little space at the top.
Pour the remaining melted chocolate on top to seal. Smooth the surface.
Refrigerate until firm, then carefully remove from the mold.