Preheat oven to 350°F/177°C. Place a silpat/silicone baking mat on 2 baking sheets. Then grease a 8-inch square cake mold.
Prepare meringue wafers: In a bowl, beat egg whites until soft peaks form. Add sugar gradually while beating continuously until stiff. Fold in cocoa, nuts and vanilla.
Spread evenly and thinly on the prepared pans.
Bake until light brown.
Loosen from the pan and slide off with a spatula while hot. Cool.
In a saucepan, combine sugar, coffee and water. Cook until thread-like.
Beat egg yolks until thick. Slowly pour in the hot syrup while beating continuously until thick and cool.
In a separate bowl, cream the butter until soft. Beat in the cream.
Add the egg yolk mixture and beat until smooth. Chill if too soft to spread.
To assemble: Layer the meringue wafers with the buttercream.
Frost all over with more buttercream.
Pipe out decorative borders then sprinkle with more nuts. Chill before serving.