Preheat the oven to 180°C.
Make sure the croissants have completely defrosted. Cut them in half lengthways, then press them into the bottom of a lined tart tin, smushing them all together. Go around the edges with croissant halves to make a rim.
Place a circle of baking paper over the middle and pour over the baking beans. Bake for roughly an hour, keeping an eye on the edges - if they brown too much, cover them with clingfilm. After 45 mins, remove the baking beans (careful, they are hot). Allow the top to brown before removing from the oven.
Grate the dark chocolate into a heatproof bowl. Heat the cream until almost boiling, then pour it over all the chocolate. Leave to sit for 5 mins, then gently fold through the butter until you have a smooth ganache.
Pour this into the centre of the croissant tart base, then leave it in the fridge to set. Bring it out to room temp before slicing and serving with a squirt of whipped cream and a sprinkling of cocoa powder.
