Start by laying a cling foil sheet over a small sized glass or ceramic tray (I used a 15x21x6 cm glass tray).You could also butter the sides and bottom to allow easy removal of the chocolate, but I really think the cling foil is the best mess-free solution. Sift the powdered milk and cocoa in a large bowl. Whisk until combined.
Add the water to a small saucepan on medium to low heat, then add the sugar and bring to boil. Leave to boil on low heat for 10 minutes from the moment it started bubbling.
Add the diced butter and the rum essence to the sugar syrup and stir with a spatula until the butter is completely melted.
Add the liquid composition over the powdered milk and cocoa and stir vigorously with a wooden spatula. You have to move fast with this composition as it hardens really quickly.
Once everything is combined, transfer the obtained paste to the glass/ ceramic tray on top of the cling foil.
Leave to cool down at room temperature, then insert in the fridge for at least 2-3 hours.
Take out of the fridge, pull the sides of the cling foil and transfer the chocolate to a cutting board. Use a large knife to cut half a centimetre slices width-wise, then each slice in half. Since I used a 15x21x6 cm glass tray, I obtained 20 slices which I cut in half, thus resulting in 40 small slices. Keep in the fridge until serving.