Line an 8x8 inch (20x20 cm) pan with parchment paper, leaving some overhang for easier lifting later on. Set aside.
Combine semisweet chocolate chips, 1/2 cup (80g) milk chocolate chips, butter and maple syrup in a heatproof bowl. Set it over a pot of simmering water, making sure the bottom of the bowl doesn’t touch the water. When the ingredients are mostly melted, gently stir them with a spatula until they all come together. Remove bowl from heat.
Mix Biscoff cookies, pretzels, marshmallows and sliced caramels in a bowl. Pour the melted chocolate over the dry ingredients, mixing with a spatula until everything is coated.
Transfer the mixture to the prepared pan and spread it as evenly as possible with a spatula, getting into every corner too. Place aside.
Take the remaining 1 cup (160 g) of milk chocolate chips and add them to the bowl in which you first melted the chocolate. Place them over simmering water to melt. Once melted, pour the chocolate over the rocky road in an even layer. Sprinkle with chopped pistachios (and a few pretzels, if desired). Cover pan with aluminium foil and place in the refrigerator to harden. This takes about 2 hours, but you can wait longer.
Take it out of the refrigerator and pull the whole rocky road from the pan. Using a sharp knife, slice it into 16 pieces, then serve. (I like to leave it at room temperature for about 15 minutes before eating, so that it's not ice cold.) Store leftover rocky road covered in the refrigerator.