Burnt sugar cake really is an act of love cause it can be an easy hit or easy miss.
Burnt sugar cake becomes hard primarily because the sugar is heated beyond caramelization, causing the sucrose molecules to break down and turn into a brittle, glass-like substance upon cooling. Overcooking the sugar syrup, or using too high a temperature, evaporates too much moisture, leading to a hard texture. Sugar should be soft boiled. Overcook and the cake will be hard/dry.
Have you ever had Burnt Sugar Cake?