Prepare the chocolate mixture: Melt margarine with almond milk, powdered sugar, cocoa, vanilla, and salt. Turn off the heat and add the couverture chocolate.
Combine with biscuits and walnuts: Crush the biscuits coarsely and mix with walnuts and the chocolate mixture.
Form the mosaic: Place the mixture on parchment paper, roll into a log shape, wrap in cling film, and refrigerate for 3-4 hours.
Serve: Once set, cut into slices and optionally dust with powdered sugar before serving.