Blitz the biscuits into crumbs using a food processor, or you can put them in a bowl or freezer bag and bash them up 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 icing, mix the water into the icing sugar. You may not need all of the water, so add it slowly bit by bit until a thick paste forms. Put about half of the icing into a separate bowl and colour it with the orange food colouring
Pour the white icing into the biscuit base around the edge, leaving a space in the centre. Then pour the orange icing into the middle
To make the chocolate ganache, warm the double cream up in a pan on a low heat, give it a stir reguarly
Take the cream off the heat when it starts to steam. 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
Pour the ganache into the biscuit base over the icing. Gently spread it out and smooth the top using a small palatte knife. You can also tap the tin very 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, melt the chocolate, split it in half and colour one half orange with the food colouring
Remove the tart from the tin and drizzle both chocolates all over the tart
Whip up the double cream with the icing sugar and vanilla extract and pipe it around the edges of the tart, add the Creme Eggs on top of the whipped cream
Store the tart in the fridge and eat within 3 days