Prepare the pan. Line a pan about 8 × 10 inches (20 × 25 cm) with parchment, leaving handles on opposite sides. For larger quantities, prepare additional matching pans as described in the notes so the layer depth stays the same.
Grind the dry ingredients. Process the butter biscuits into fine crumbs. Finely grind the walnuts separately in short pulses, stopping before they become a paste. Combine thoroughly in a large heatproof mixing bowl. Finely chop the chocolate portions and keep the dark-mixture and topping amounts separate.
Make the syrup. Put the granulated sugar, vanilla sugar, fine salt, and measured water into the saucepan. Heat while stirring until dissolved, then bring briefly to a boil. Remove from heat as soon as the sugar is dissolved and the mixture has boiled; do not simmer it down to caramel or a thick candy syrup. Be careful of hot syrup splashes.
Melt in the butter. With the saucepan off the heat, stir in the butter pieces until fully melted and the liquid is uniform.
Combine the base. Pour the hot liquid over the cookie-and-walnut mixture and stir thoroughly, scraping the bottom and corners. It should be evenly moist, soft, and formable, with no loose dry pockets or free liquid. Keep working while it is warm; do not add unmeasured water or milk.
Divide and color. Weigh the warm base into two equal portions. Keep one pale. Stir the listed dark-mixture chocolate into the other until completely melted and evenly incorporated. If the chopped chocolate needs help, melt that measured portion gently in a dry heatproof bowl, then mix it in without adding liquid. Divide the finished dark portion into two equal portions.
Make three layers. Press the first dark portion evenly across the pan, including the corners. Add the full pale portion, distribute gently, and press level without smearing the layers together. Finish with the remaining dark portion and smooth the surface. Keep the order DARK–LIGHT–DARK, with each layer uniform from edge to edge. At larger scales, distribute each corresponding layer evenly among the matching pans.
Chill the base. Refrigerate for about 30 minutes so the top firms before glazing. Keep the pan level.
Prepare the topping. During the latter part of the initial chill, gently melt the separate topping chocolate in a dry heatproof bowl over barely simmering water, without letting the bowl touch the water, or microwave at low power in short bursts, stirring often. Stir in the measured sunflower oil until smooth. Let cool for about 3–5 minutes while remaining pourable; keep water and steam out of the chocolate.
Glaze. Spread the topping in a thin, even layer over the chilled base. Smooth gently with an offset spatula, dividing the topping evenly among matching pans when scaling.
Chill thoroughly. Return to the refrigerator at 40°F (4°C) or below for at least 3 additional hours, until the entire slab is firm. Allow longer if the center is still soft; overnight chilling is also suitable.
Cut cleanly. Use the parchment handles to lift the chilled slab onto a board. Let stand for about 10 minutes to take the hard refrigerator edge off the glaze. Warm a long knife in hot water and dry it completely before cutting small rectangles. Clean, warm, and dry the blade again as needed. Divide the selected total yield evenly among the matching pans when scaling, then return the bars to refrigerated storage.