Prepare a baking dish by lining it with parchment paper.
In a saucepan, melt the dark chocolate, milk chocolate, and butter over low heat, stirring constantly.
Once melted, remove from heat and stir in the vanilla extract.
Allow the mixture to cool slightly before adding the nuts and marshmallows.
Gently fold in the mini marshmallows and nuts until evenly coated.
Pour the mixture into the prepared baking dish and spread it evenly.
Sprinkle desiccated coconut on top, if using.
Refrigerate for at least 2 hours until set.
Once set, remove from the dish and cut into squares.
Serve chilled or at room temperature.