In a double boiler (or a regular saucepan on medium-low heat), melt the butter with the sugar and cocoa. Add egg and stir to cook and thicken. Remove from heat and stir in the crumbs, coconut and nuts. Press firmly into an a parchment-lined 8x8-inch pan. Cut the Easter creme eggs in half lengthwise and place cut side down on the base. Refrigerate while you make the frosting (second layer).
Cream butter, cream, custard powder and icing sugar together well. Beat until light. Spread over bottom layer. Melt chocolate and butter over low heat or in a bowl in the microwave. Cool slightly, pour over second layer and refrigerate until set. Cut into 16-20 squares.