Blitz the biscuits into crumbs using a food processor, or you can put them in a bowl or freezer bag and bash them up gently with the end of a rolling pin
Place the biscuit crumbs into a bowl. Add the cocoa powder and mix it in. Then add the melted butter and mix it in
Press the biscuit mixture into the base and sides of a 9" loose bottomed tart tin. An American cup measure, or a shot glass can help you to evenly press it around the sides and base
Put the biscuit base in the fridge while you make the filling
To make the mint icing, mix the water and peppermint extract into the icing sugar. You may not need all of the water, so add it slowly bit by bit until a thick paste forms
Pour the mint icing into the biscuit base and spread it out to cover the base
To make the chocolate ganache, warm the double cream up in a pan on a low heat until it steams, give it a stir reguarly and do not allow it to boil
Take the cream off the heat. Add the chocolate to the cream and leave it for a couple of minutes, then stir until it all melts together. Then stir in the pinch of salt, if using
Pour the ganache into the biscuit base over the mint icing. Gently spread it out and smooth the top using a small palatte knife. You can also tap the tin gently on the work surface to even out the ganache and remove any air bubbles
Leave in the fridge for at least 4 hours, or ideally overnight, to set
For decoration, whip up the double cream with the icing sugar and vanilla extract
Remove the tart from the tin and pipe the whipped cream around the edges, add the After Eights all around the edge and dot some mint leaves around in the cream too. Then use a sieve to dust the tart with cocoa powder
Store the tart in the fridge and eat within 3 days