Bring the heavy cream to a boil and pour over the chocolate. Let the mixture sit without stirring for 1 minute. Stir with a rubber spatula until the chocolate has melted. Mix in the butter and stir until it has melted. Cover and allow to firm for 45 minutes at room temperature.
Meanwhile, line a half sheet pan with a nonstick rubber mat. Place a 25cm / 10in by 38cm / 15in by .5 cm / .25 in Plexiglas frame inside the sheet pan. Place the sheet pan in the freezer.
Pour the ganache into the prepared frozen frame and spread out evenly using an offset spatula. Harden under refrigeration.
Line a half sheet pan with a nonstick rubber mat and refrigerate.
Once the ganache has hardened, flip the ganache onto another nonstick rubber mat before cutting it. Place it back on the cold sheet pan and peel off the top rubber mat. This will make the process easier, since the ganache won’t stick to the new rubber mat as much. Using a 5-cm / 2-in square cutter, cut the ganache out, dipping the square cutter in warm (not hot) water each time to get a clean cut.
Once the ganache is cut, lift each square using a small offset spatula and transfer to the second chilled sheet pan lined with a nonstick rubber mat. If the uncut ganache starts getting too soft, return it to refrigeration. Cut it once it firms up again.
Transfer to an airtight container and reserve at room temperature. Discard any leftover ganache after 5 days.