Preheat oven to 325 degrees.
Line two 9 inch round cake pans with parchment or waxed paper.
Mix cocoa and flour in small bowl; set aside.
Break up almond paste into small pieces and place in food processor bowl.
Add 1/4 cup sugar.
Cover and pulse until mixture resembles coarse meal.
Add water and pulse until smooth.
Add cocoa mixture and pulse until combined.
Turn mixture into large bowl.
In large mixing bowl on high speed beat egg whites with cream of tartar until foamy.
Gradually add remaining 1/4 cup sugar, beating until soft peaks form.
Stir 1/4 of the meringue into the cocoa almond mixture to lighten it.
Fold in remaining meringue, using a large rubber spatula.
Spread mixture in prepared pans.
Bake 18 to 20 minutes or until center of each layer is firm to the touch.
Cool completely in pans on wire racks.

Chocolate Mousse:
Microwave chocolate on high for 1 minute.
Stir until smooth.
Keep chocolate slightly warm.
Combine water and 3/4 c sugar in small saucepan.
Cook over low heat, stirring with a metal spoon until sugar is dissolved, about 3 to 4 minutes.
Dip a clean pastry brush in water and wash down any sugar crystals on the inside of saucepan.
Increase heat to medium and bring mixture to a boil.
Cook, without stirring, until the syrup registers 240 degrees on candy thermometer, about 5 to 8 minutes.
When syrup cools to 230 degrees, start beating the egg whites.
In large mixing bowl on high speed beat egg whites until foamy.
Gradually add remaining 1/4 c sugar, beating until soft peaks form.
Pour sugar syrup into beaten egg whites in a thin, steady stream, using medium speed.
Increase speed to medium-high and beat 5 minutes more or until the meringue is cool and forms stiff peaks.
Place butter in large mixing bowl.
Add chocolate to butter in several additions, beating with mixer until creamy.
Stir about 1/2 c meringue into chocolate mixture to lighten it.
Then gently fold remaining meringue into chocolate mixture in three additions.

Assembly:
Line work surface with parchment paper.
Dust with powdered sugar.
Invert each layer onto prepared parchment and remove parchment rounds.
Slide two wide metal spatulas under one of the layers and transfer to serving plate.
The layers are very fragile.
Any tears will be covered by mousse.
Reserve about 1 1/2 c mousse.
Spread the remaining mousse over layer.
Top with other layer.
Refrigerate assembled dessert and reserved mousse for 30 minutes to firm up.
Spread reserved mousse on top and sides of dessert.
Refrigerate for at least 4 hours before serving.
Dust with powdered sugar before serving.