1. Combine milk and marshmallows in a pan over a low heat. Stir until marshmallows have melted.
2. Add coffee and liqueur, then stir until smooth and combined. Cool completely.
3. Whip the topping until firm peaks form and fold into the marshmallow mixture.
4. Pour into the tart shell and chill for two hours or until set.
5. Whip the topping and sugar for the garnish layer in a mixer until firm. Place in a pastry bag fitted with a plain round piping tip. Pipe small mounds of topping as desired to cover the mousse.
6. Dust slightly with cocoa.