Heat thef water together with the demerara sugar and vanilla sugar. Simmer for 4-5 minutes for a softer chocolate, or 8-10 minutes for a firmer, more brittle texture.
Add the butter and mix until it melts. Then, add the rum essence to preserve its stronger aroma.
Combine the milk powder and cocoa powder in a bowl. Pour the warm syrup and mix everything with a spatula or wooden spoon.
Mix well until everything is well incorporated and no traces of milk or cocoa powder are visible.
Pour the mixture into a baking dish lined with plastic kitchen foil, ensuring it's level.
Place the tray in a cool place or in the refrigerator for at least two hours, or preferably overnight.
Once firm, you can slice it into any shape and size you desire.
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Notes
You can use vegan milk powder and vegan butter alternatives to make a plant-based version.Replace the demerara sugar with a sugar-free alternative like erythritol or stevia.Simmer the sugar syrup for 4-5 minutes for a softer chocolate, or 8-10 minutes for a firmer, more brittle texture.