Add the chocolate and vanilla extract to a large heatproof bowl; set aside.
In a heavy bottomed pot set over a medium-low heat, combine the cream, milk, sugar, cocoa powder, and salt. Whisking often, continue to heat the mixture until bubbles form around the edges or the temperature on a digital thermometer hits 82-87°C. Remove from the heat and immediately pour over the chocolate/vanilla. Leave to sit for 1 minute.
Starting in the centre of the bowl, use a whisk to stir the melted chocolate into the cream. Once the centre of the mixture is entirely combined then begin to work through the rest of the mixture. By the time it's entirely combined it should be silky and smooth.
Pour the mixture through the sieve into the measuring jug. Pour the mixture into the molds and place in the freezer for 30-minutes, or until semi-frozen.
When partially frozen, sprinkle a generous handful of the crumbs onto the base of each mold, about 2cm worth - keeping the leftover crumbs in a bag. Return the molds to the freezer until completely solid.