Makes one 9-inch cake; serves 12 to 16
Prepare chocolate mousse and refrigerate as instructed for at least 2 hours and up to overnight before making the cakes.
Preheat oven to 350 degrees.
Grease, flour, and line two 9-inch cake pans with parchment paper and set aside.
Melt chocolate, stirring occasionally, in a microwave or over a pot of barely simmering water. Set aside.
In a standing electric mixer, beat butter until light and smooth. Add brown sugar, molasses, and eggs and continue to beat until very light and fluffy. Beat in vanilla, sour cream, and coffee. Add baking soda and salt and beat briefly to combine.
Slowly add flour, beating until smooth. Add melted chocolate and whisk briefly to combine. (The batter will be very thin -- don't worry!)
Divide batter evenly among cake pans and bake until a skewer inserted in centers of cakes comes out clean, about 30 minutes. Cool cakes in their pans on a wire rack, 10 minutes. Invert cakes, one at a time, onto a large plate, then invert onto cooling racks to finish cooling.
Once cakes have cooled, cut each layer in half horizontally and carefully transfer one half of one of the layers to a cake plate. Tuck waxed paper or parchment under edges of cake to keep plate clean while filling and frosting layers.
Brush tops of three layers each with 1/4 cup Grand Marnier. Place one-third of the chilled mousse onto top of cake layer on plate and gently spread it just to the edges in one even layer. Top with another cake layer; repeat until you have used all the mousse. For final cake layer, use a domed cake layer, arranging it domed side up. Place toothpicks into the cake in several places if the layers appear unstable, then cover cake with plastic wrap and refrigerate for several hours before frosting. (Cake may be made up to this point up to 1 day in advance.)
Once cake seems to have set and is more stable, make ganache as recipe instructs and set aside until cooled and just beginning to firm up, about 1 hour.
Remove cake from refrigerator and spread a thin layer of ganache all over entire cake, spreading with an offset spatula to smooth ganache on all sides. Refrigerate for ganache to set, about 10 minutes. Remove cake from fridge and repeat with another layer of ganache; depending on how firmly ganache has set, you may be able to completely frost cake with remaining ganache or you may need to apply half of it and then repeat to use all the ganache. Once completely frosted, refrigerate cake for at least 20 minutes and up to overnight.
Before serving cake, let it come to room temperature for at least 30 minutes before serving. Cake is best served at room temperature and is most easily cut with a warm, sharp knife. Serve with raspberries, fresh mint and confectioners' sugar, if desired.