In a bowl, stir together graham crackers, coconut, ground almonds, cocoa powder and sugar.
Drizzle with butter and add egg, stirring until combined.
Press into a parchment paper-lined 9-inch square cake pan.
Bake in a 350° oven until firm, about 10 minutes.
Let cool in pan on wire rack.
In bowl, beat together butter, custard powder and vanilla.
Beat in powdered sugar alternately with milk, making 3 additions of sugar and 2 of milk, adding up to 1 teaspoon more milk if too thick to spread. Spread over cooled crust. Refrigerate until firm, about 1 hour.
In a heatproof bowl, over saucepan of hot water, not boiling water, melt chocolate with butter; spread over filling. Refrigerate until chocolate is almost set; about 30 minutes.
With tip of knife, score into bars; refrigerate until chocolate is completely set, about 30 minutes. Cut into bars.