In a saucepan, bring 278ml of heavy cream (thicken cream) to 80C (176F), make sure not to bring it to boil, otherwise you will burn the chocolate later on (smell very bad).
Then pour the warm heavy cream into the chopped chocolate bowl, then slowly stirring from inside out by a hand mixer.
Add cut unsalted butter and keep stirring until the chocolate ganache completely smooth and make sure there is no lumps.
At this point you can add rum in and give it a stir (this step is just optional as I mention above).
Sift the mixture into a tray (I use the tray with dimension 25cm x 18cm), but you can use a popular dimension 20cm x 20cm with silicon paper on it, this step to me is very important to make sure there is nothing left other than the Ganache mixture, I experience sometimes I found a very tiny piece of paper from butter wrap and you don't want it, sometimes it helps to prevent the bubbles in a mixture. A extra step but I always feel good at it.
Gently tap the tray on the bench will help the chocolate mixture running evenly and smoothly.
Place the tray into freezer about 3 hours to set.
Peel off the parchment paper and cut the slab on a board, 3cm x 3cm will be a good idea size to start with.
Sprinkle coco powder on top, decoration with golden leaves or leave it blank. If you would like to have all sides with cocoa powder, you can dip in cocoa powder to coat all sides.