Mix milk, chocolate, cocoa and vanilla sugar in a bowl until the dry mass becomes homogenous. The bowl should be large enough, so the other ingredients can be added.
Pour sugar into 300 ml of water, bring the mix to boil and let it simmer for nine minutes. Take it off the stove and slice butter into it. Let the butter melt and bring the mix almost to boil. Pour the melted mix into the prepared dry mass and mix until it becomes homogenous. Mix thoroughly until the mass becomes smooth, but not more than a minute because it hardens fast. Pour the mix into a baking tin, which you lined with aluminium foil beforehand. The standard baking tin size is appropriate for this amount of chocolate.
When the chocolate hardens (in 3 to 4 hours, or you can leave it overnight), slice it into 4 x 4 cm pieces. Such pieces are easier to take form the tin by lifting the foil, so they can be cut into even smaller pieces. The chocolate improves in taste as it matures.