To make the meringues:.

1. Preheat the oven to 300 degrees Fahrenheit.

2. Mix the cocoa powder and the powdered sugar.

3. Whip the egg whites until stiff (about 5 minutes). Halfway through, add 60 grams of granulated sugar to help them keep their shape.

4. When stiff, add the remainder of the granulated sugar at low speed.

5. Fold in the cocoa sugar mixture with a spatula gently.

6. Prepare two cookie sheets with silpats or wax paper.

7. Spoon half of your meringue batter into a pastry bag or a ziploc bag with the corner cut off.

8. Pipe your meringue batter onto a cookie sheet in three ovals 6 inch long and 4 inch wide.

9. Fill the other cookie sheet with long straight lines of meringue batter.

10. Cook in the oven for 40 to 50 minutes, until the meringue is hard but not burnt. Check the meringue frequently to ensure it does not burn.

11. Remove from the oven, and when cool, cut the long strips into one inch long pieces.

To make the mousse:.

1. Melt the chocolate in a water bath.

2. Remove from the heat and beat in the butter (room temperature, not melted). The consistency should be like pudding. Fold in the powdered sugar.

3. Add the egg yolks, making sure the mixture is relatively cool so that it is ready to be mixed with the egg whites.

4. Beat the egg whites until stiff, adding 20 grams of sugar half-way through to keep them firm.

5. Pour the chocolate mixture on top of the egg whites, and fold it carefully together, rotating the bowl a quarter turn after every fold. You want to be very gently to maintain as much of the air as possible.

6. Put the mousse in the fridge for an hour to chill.

To assemble the cake:.

1. On a serving dish, lay one oval meringue disk.

2. Cover the meringue disk with chocolate mousse.

3. Lay another meringue disk on top of the mousse.

4. Cover it with more mousse and repeat with the last meringue disk.

5. Cover the entire cake with chocolate mousse and then decorate it to fully cover most of the mousse with the inch-long meringue pieces.

6. Dust with a little powdered sugar.

7. Refrigerate before serving. The cake tastes best if made the day before.