I have one from Alice Medrich's class that is really rich and delicious, I have used it as a filling and it held up well but I also refrigerated the cake overnight, not sure you planned to do that.
Chocolate Marquis
10 oz. good quality semi sweet chocolate cut into small pieces
4 oz. (1 stick) sweet butter, cut into small pieces
2 teaspoons instant coffee dissolved in 2 teaspoons very hot water
4 large eggs, separated
1/8 teaspoon cream of Tartar
2 Tablespoons granulated sugar
1) Melt chocolate and butter together in a medium bowl placed in a pan of barely simmering water. stir from time to time to hasten melting. When chocolate is melted and smooth, whisk in dissolved coffee and the egg yolks. Remove from heat and set aside.
2) In a clean dry bowl, beat egg whites and cream of tartar at medium speed until soft peaks form. Gradually sprinkle in the sugar, beating at high speed until stiff but not dry. Fold one fourth of egg whites into chocolate mixture to lighten it. Fold remaining whites into mixture until completely incorporated. Scrape mousse immediately into a prepared mold and refrigerate for at least 4 hours before unmolding. Or, scrape into a bowl, cover and let chill till set. Stir mousse and fill a piping bag with a large round tip and pipe filling in spiral from center out, on bottom cake layer. Cover with top cake layer.
Mousse can be made 2 days ahead and kept refrigerated, or frozen for up to 2 months.
Shirley, that sounds delish!!! It will have to go my list of things to do!! Since getting on this site, it seems like everyday I find something new I want to do!
Not sure about how sturdy cause I never tried it in a cake is Hersheys Chocolate Mousse. I got the recipe online...i was trying to find it again....I am at work and the recipe is in my recipe box 
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