Line your square pan with parchment paper, leaving some overhang for easy removal.
Chop the dark chocolate into small pieces to help it melt evenly.
Gently melt the chocolate in a heatproof bowl over a double boiler or in short bursts in the microwave, stirring until smooth and glossy. The chocolate should feel warm and silky to the touch.
Add the butter to the melted chocolate and stir until it’s completely incorporated and the mixture is shiny and smooth.
Stir in the sweetened condensed milk and vanilla extract until the mixture is uniform, glossy, and thick.
Fold in the crushed graham crackers and toasted nuts if using, mixing gently until evenly coated and the mixture feels thick but still workable.
Pour the mixture into the prepared pan, spreading it out evenly with a spatula or the back of a spoon. Press down lightly to compact the mixture.
Place the pan in the fridge and chill for at least 2 hours, or until the bars are firm and glossy on top.
Lift out the firm block from the pan using the parchment overhang, then cut into squares or bars with a sharp knife, wiping the blade clean between cuts for neat edges.
Serve immediately or store in an airtight container in the fridge for up to a week for best freshness.
