Line the base and sides of the cake tin with greaseproof paper - use oil to stick as butter will chill too much and make removing the pie difficult.
Using a food processor blitz 400g Maltesers into crumbs.
Make the crust: put the melted butter into a bowl and add the Malteser crumbs, then mix well. Press into the cake tin, and push down evenly with the back of a spoon until the bottom and sides are completely covered- come right the way up to the top of the tin. Set aside to chill in the fridge.
Make the filling: in a medium pan add the cream and malt powder and heat until nearly boiling. Remove from the heat and add the chocolate, stirring until everything has melted together and has a smooth, silky consistency. Set aside to cool a little before pouring into the tin - the mix will come right up to the top. Leave to set for a minimum of 4 hours, ideally overnight.
To serve: remove the pie from the tin and transfer onto a serving plate. Whip the double cream and vanilla together to soft peaks. Spoon over the chocolate filling, evenly covering the surface. Bash the remaining Maltesers and sprinkle over the top.
