Prepare everything before making the syrup.
Line a medium casserole dish with cling film. Let the butter come to room temperature and chop into small cubes. Weigh the cocoa into a small bowl and set aside. Place the powdered milk in a large bowl. Roast the nuts, if using, in a pan without fat while shaking often and let cool.
To make the syrup place water and sugar into a medium pot. Bring to a boil and simmer stirring until the sugar dissolves and a light syrup is formed.
Take the syrup off the heat, add the cocoa powder, place the pot back on the stove and let it bubble just one more time.
Take off the heat and add the cubed butter, stirring well to incorporate.
Immediately pour the liquid over the powdered milk and stir vigorously.
Pour the mixture into the prepared casserole dish and level nicely.
If using nuts, you could either mix them into the chocolate before adding the mixture to the dish or sprinkle them over the chocolate in the dish.
Cover with cling film, let cool completely on the counter and then place in the fridge until set. It will take about 2 hours.
Cut into squares or rectangles before serving.