Preheat oven to 325°F (165°C) and line an 8x8 inch baking pan with parchment paper, leaving an overhang.
Toast the almonds, walnuts, and cashews on a baking sheet for 10 minutes or until fragrant.
In a large bowl, combine the toasted nuts, honey, vanilla extract, and fine sea salt, stirring until all nuts are evenly coated.
Transfer the mixture to the prepared baking pan and press down firmly using a spatula or the bottom of a measuring cup to create an even layer.
Bake for 15 minutes or until the edges are golden brown.
Remove from oven and allow to cool completely in the pan for at least 1 hour.
Melt the dark chocolate in a double boiler or microwave in 30-second intervals until smooth.
Drizzle or spread the melted chocolate over the cooled nut layer and sprinkle immediately with flaky sea salt.
Refrigerate for 20 minutes to set the chocolate, then lift the bars out of the pan and slice into 12 rectangular pieces.