Melt the unsalted butter in a double boiler, crush the cookies, and line the bottom of the mold with parchment paper for easy removal (if using a springform pan, you can skip the parchment paper).

Mix the butter and crushed cookies, press into the mold, and refrigerate for 30 minutes.

Heat the chocolate and heavy cream in a double boiler until the chocolate melts, mix well, pour into the mold, and refrigerate for two hours. Run a spatula around the inside of the mold, invert onto a shallow plate, then invert onto another plate.

I used 70% chocolate, and after adding 20 grams of sugar, it was slightly bitter. You can choose your favorite chocolate, and adjust the sugar to taste. I ran out of digestive biscuits at home, so I used plain cookies, which are best as they don't overpower the flavor.