Grease a 20cm pie dish. Place the chocolate digestives in a ziplock bag and use a rolling pin to crush them. Tip the crumbs into a small bowl, add the melted butter and stir well. Press the biscuit mixture into the base of the dish in an even layer, then pop the dish in the fridge.
Preheat your oven to 180°C. Place the dark chocolate and butter in a saucepan. Stir over a low heat until the chocolate and butter have melted. Stir in the vanilla extract, then set the pot aside.
Whisk the eggs and the castor sugar together until pale and combined, about 2 minutes. Slowly fold the chocolate mixture into the egg mixture and stir until combined. Pour the chocolate mixture into the pie dish and bake for 20 minutes.
While the chocolate is baking prepare the meringue topping. Whisk the egg whites until foamy. Add the salt and cream of tartar and whisk until soft peak stage. Add the castor sugar one tablespoon at a time while whisking, until it is fully incorporated and the meringue has stiff peaks. Sift the cocoa over the meringue and stir a few times (don't overdo it!) to give a streaky effect.
Pile the meringue onto the hot chocolate filling. I like to leave a bit of space around the edge to allow the meringue to spread a little. Bake for 25 minutes, the centre will be wobbly. Set aside until completely cold and the filling has set.