Crush the biscuits into fine crumbs using a food processor, then transfer to a bowl.
Pour the melted butter over the biscuit crumbs and stir until well combined, creating a moist, sandy mixture.
Press the biscuit mixture evenly into the base and up the sides of a 23cm tart pan, then refrigerate for 15 minutes to set the crust.
Gently melt the dark chocolate in a microwave in short bursts or over a double boiler, stirring until smooth and glossy. Let it cool slightly.
In a bowl, blend the peanut butter, vanilla extract, and a pinch of sea salt until creamy and smooth.
Fold the cooled, melted chocolate into the peanut butter mixture until fully combined and smooth.
Pour the chocolate peanut butter filling into the chilled crust, spreading it evenly with a spatula.
Refrigerate the tart for at least 2 hours, or until the filling is firm and glossy.
Just before serving, sprinkle flaky sea salt on top for a flavor contrast.
Use a sharp, warm knife to slice the tart cleanly, dipping it in warm water between cuts.
Serve the tart at room temperature or chilled, and enjoy the rich, creamy, and crunchy textures in every bite.