Preheat the oven to 180 C / Gas 4. Grease and line a 22cm square (9 in) tin with baking parchment.
For the biscuit base:
Whiz the digestives in a food processor to fine crumbs, then transfer to a bowl. Add the hazelnuts, cocoa and sugar, stir to combine then mix in coconut.
Stir in the melted butter, then add the beaten eggs, stirring until combined. Transfer mixture to the tin and press gently the mixture gently down with the back of a spoon to compact.
Bake in the preheated oven for 15 minutes. Remove from oven and leave to cool.
For the custard buttercream:
For the buttercream, whisk butter until pale and fluffy, then mix in custard powder and vanilla. Add icing sugar mix until crumbly then add the milk and mix until you have a pale, mousse-like filling. Spread over the cooled biscuit base and chill for about an hour.
For the topping:
Melt the chocolate and butter together, then whisk in the coffee, it may separate at first, but just keep whisking until the mixture is smooth and glossy. Smooth chocolate over the top of the buttercream and chill until set.
Cut into bars, the size of your choice, and enjoy!