1. Begin by preparing the sugar decorations. In a dry saucepan, heat the sugar over a gentle heat until it melts and turns a golden-brown color. It is important to keep swirling the pan so that the sugar does not burn, but do not stir it. Take care towards the end as the sugar can suddenly burn quickly so remove it from the heat as soon as it is a deep golden color. Allow to cook for one minute, which will allow the caramel to thicken and make better shapes. Using a spoon, swirl the hot sugar in two spiral patterns on a silicone mat or parchment paper and leave to set (which will only take a few minutes).
2. Pour the hot coffee into two cups or heatproof glasses. Steam or froth the milk until hot and very foamy — you want a ratio of approximately 50–50 steamed milk and foam. Add a little vanilla syrup or vanilla extract to each drink, stir, and pour the hot milk on top of the coffee. Top the drinks with marshmallows and place a sugar decoration on top of each one. Serve at once with a spoon so that you can ‘crack’ the topping into the coffee and stir it in.