In a food processor or blender, crush Biscoff cookies until they are finely ground and transfer to a bowl. Mix them with melted butter until well combined. Divide the mixture into 4 mini pans**. Press the cookie mixture into each pan, working up the sides a little bit, use a spoon to pack it tightly. Place in the fridge while making the filling.
Place finely chopped dark chocolate bar and Biscoff spread in a glass bowl, and put it over a small pot with just-boiled water. Don't let the bottom of the bowl touch the hot water. Let it slowly heat up and melt. Mix it through a few times. Once the chocolate is melted, slowly and gradually, using a spatula or spoon, mix in unsweetened oat milk. Stir until combined into a smooth ganache.
Pour the ganache over chilled cookie crust and smooth it out evenly. Place in the fridge for about 1 hour to set.
Once the ganache has set, pour over melted Biscoff spread and smooth it out. Place back to the fridge to set for about 30 minutes.
Once set, decorate, and serve! I served them with coconut whipped cream, crushed Biscoff cookies, grated chocolate, and sea salt.