Melt butter. Add in sugar and egg. Fold in cocoa powder, flour, salt, coffee. Alternating with sour cream until ingredients are combined. Bake in desired baking pan until tester comes out clean. Cool completely before using to build mousse.
Whip whipping cream, cold water and salt until stiff. Divide into 3 separate bowls.
Add melted dark chocolate in one bowl with whipped cream. Do the same with melted milk and white chocolate. Make sure chocolate is warm but not too hot to melt whipped cream.
To build mousse, first put in brownie base layer. Followed by white chocolate whipped cream, then milk chocolate and the dark chocolate cream last. Allow to chill and set in between layers to get clean, distinct layers.
To garnish, dust with cocoa powder and powdered sugar.
