Place two layers of clingfilm into a small pie dish (approx. 5 × 20 cm). Boil the cream then remove the pan from the heat. Break the chocolate into chunks, add to the pan and stir to melt. Add the Cognac.
Scatter the pistachios over the bottom of the pie dish, add approx. 3 tbsp of the chocolate mixture, level off. Add a layer of halved Petit Beurre and continue with this sequence until everything has been used up, ending with chocolate mixture (should create approx. 4 layers). Cover and refrigerate for approx. 3 hrs.
Remove the cake from the tin, remove the clingfilm. Slice to serve.