This cake is rich, each serving is 487 cal. and it is time consuming but it is worth making it.
I found this recipe in the First Magazine  a few years ago.

Work Time 2 hours
Total Time 4 hours 

Serves 24

• 2-1/2                  tsp instant espresso
•                           or coffee
• 2                        cups cake flour
• 3 -1/4                 cups sugar
• 3/4                     cup unsweetened cocoa
• 1-1/4                  tsp. baking soda
• 1/4                     tsp baking powder
•                           pinch salt
• 8                        eggs
• 1                        cup sour cream
• 1/2                     cup butter
• 2-1/2                  tsp vanilla
• 15                      tbsp butter
• 12                      oz semisweet chocolate
• 3                        oz unsweetened chocolate
• 1 1/2                  cups whipping cream

Instructions

1. Heat oven to 350 degrees.
2. Grease 2 – 9 inch cake pans.
3. Dissolve espresso in 1 cup hot water.  Cool
4. Combine flour, 1-1/2  cups sugar, cocoa powder, baking soda, baking powder and 3/4 tsp. salt.
5. Add espresso, 2 eggs, the sour cream, butter and 1 tsp vanilla.
6. Beat on medium speed 2 minutes.
7. Pour into pans.
8. Bake until cake pulls away from sides of pan, 25 to 30 minutes.
9. Cool on the racks 10 minutes.
10. Remove from the pans.
11. Cool completely.

Chocolate Mousse

1. Chop 9 oz semisweet chocolate and the unsweetened chocolate.
2. Melt 12 tbsp  butter over low heat.  Turn off the heat, stir in chopped chocolate until smooth.
3. Separate remaining 6 eggs.
4. Gradually beat 3/4 cup sugar into yolks.  Slowly beat in 1 tsp vanilla and the chocolate mixture.
5. Beat the egg whites with a pinch of salt until they hold soft peaks.  Gradually beat in 6 tbsp of the sugar and continue beating to stiff peaks.
6. Gently fold the whites into the chocolate mixture.
7. Beat 3/4 cup of cream until almost stiff and fold into the chocolate mixture.
8. Set aside 4 cups of this mousse.
9. Refrigerate the remaining mousse.

To Assemble the cake

1. Cut the cakes in half horizontally to make 4 thin layers.
2. Put two layers on separate plates.  Spread each with 2 cups of reserved mousse.  Chill until firm, about 20 minutes or freeze 10 minutes.
3. Top each with a second cake layer and refrigerate. 
4. Beat remaining 1/4 cup cream with 1/4 cup sugar and 1/ tsp vanilla until stiff.
5. Spread the whipped cream over one of the layered cakes.  Chili until firm, about 30 minutes.
6. Top with the second layered cake .  Chili until firm, at least 20 minutes.
7. Stir the remaining mousse to soften and spread over top and sides of cake.
8. Melt the remaining 3 oz semisweet chocolate with the remaining 3 tbsp butter over low heat.
9. Drizzle the melted chocolate over the top and around the edge of the cake.