Set aside a little of the milk in a cup and dissolve the starch with 1 tbsp powdered sugar in it. Put the rest of the milk with the sugar in a pot to come to a boil.
Stir until the sugar melts, pour in the dissolved starch. Stir and once it thickens, add the vanilla. Divide it up into 2 separate bowls, one in which you've put the grated white chocolate, and the other in which you've put grated dark chocolate beforehand. Add 1 tbsp butter into each.
To the bowl with the dark chocolate, add the dissolved coffee with 1 tbsp hot water.
Stir both creams simultaneously in each of their respective bowls, while keeping both bowls in your hands so that you can make swirls from both colors.
Once it cools, you can decorate the cream with grated white chocolate and chocolate buttons.