Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans.
Melt the butter, stir in the cocoa powder, and set aside to cool.
Using an electric mixer, beat the eggs for 3 minutes, then gradually add the sugar, beating 2 to 3 minutes more. Stir in the butter/chocolate mixture. Gently fold in the flour, walnuts and salt; just until combined.
Pour the batter into the prepared pans. Bake for 20 to 25 minutes, or until a cake tester inserted in center of cake comes out clean.
Remove the cakes from the oven and cool in the pans for 5 minutes, then turn onto racks to cool completely.
Whip the cream until it begins to thicken, 2 to 3 minutes. Gradually beat in the brown sugar, confectioners' sugar and espresso powder or coffee. Continue beating until stiff peaks form.
Spread the whipped cream atop one cake layer, top with the second layer, and use the remaining cream to completely cover top and sides of cake. Garnish the top of the cake with chocolate chips or sprinkles. Refrigerate at least 1 hour, or overnight, befor