Line a 20cm cake tin with clingfilm, allowing plenty to hang over the sides. Grease the bottom and sides well with vegetable oil. Grease a spatula or large spoon. Sift together 2 tbsp cocoa powder and the icing sugar and set aside.
Pour 110ml cold water into the bowl of a stand mixer. Add the gelatine and set aside to allow the gelatine to absorb the liquid. Meanwhile, put the granulated sugar and golden syrup into a small heavy-bottomed saucepan and cover with another 110ml cold water. Place on a low heat and gently stir to combine, using a wet pastry brush to brush down any sugar on the sides of the pan. Continue to cook on a low heat until the sugar has dissolved, then turn up the heat to bring it to the boil. Once the syrup reaches 130°C-140°C – at least 5 minutes – take the pan off the heat and leave it to cool for 1 minute.
Using a stand mixer on a medium-low speed, begin to whisk the gelatine mixture. Slowly add the syrup, pouring it down the inside of the mixer bowl and not directly on to the whisk. After all the syrup has been added, add the vanilla extract, turn the speed up to medium high and continue to whisk for 8-10 minutes, until the marshmallow mix becomes really thick and has cooled. Once it starts to form stretchy strands on the surface, add in the remaining cocoa powder and mix briefly to combine. Using your greased utensil, spoon the marshmallow into the prepared tin, dust 1 tsp of the cocoa and icing sugar mix over the entire surface and cover lightly with the overhanging clingfilm. Leave to set in a cool place for 2 hours, until the top feels firm when pressed.
Set a heatproof bowl over a pan of barely simmering water. Break up the dark chocolate and add to the bowl to melt, stirring until smooth. Remove the pan from the heat but keep the bowl over the water and set aside. Lightly dust a worksurface with 1 tsp of the cocoa and icing sugar mix and turn the marshmallow out on to it, cocoa side down. Peel off the clingfilm. Using a sharp knife dipped into hot boiled water, cut into 40 even-sized cubes. Toss each in the last of the cocoa and icing sugar mix, making sure every side is coated.
Working with one marshmallow at a time, brush off any excess coating, then dip one side into the chocolate, allowing any excess to drip off. Transfer to a baking tray lined with baking paper, chocolate side up. When they are all done, refrigerate for 15 minutes or until set.