Preheat oven to 350 degrees Fahrenheit. Spray a long rectangle tart pan with baking spray.

In a food processor, cream margarine and sugar. Add flour and pulse only until incorporated. Add coffee grounds and pulse to combine.

Transfer shortbread dough to a prepared tart pan and pat it into the pan. Using your fingertips, press the crust evenly into the tart pan and up the sides. Sprinkle it with a pinch or two of sugar to make the job a bit easier. Place a sheet of Gefen Parchment Paper into the tart shell and fill it with pie weights or dried beans. Bake the crust for 15 minutes.

Remove from oven and carefully remove the parchment and pie weights or beans. Return crust to the oven and bake for an additional 10 minutes. Let cool.

In a small saucepan, combine topping and coffee; stir to dissolve. Heat just to a boil. Remove from heat.

Add chopped chocolate and stir to melt the chocolate and emulsify the mixture. Let stand for five minutes to cool.

Pour into cooled crust.

Combine milk and marshmallows in a pan over a low heat. Stir until marshmallows have melted.

Add coffee and liqueur, then stir until smooth and combined. Cool completely.

Whip the topping until firm peaks form and fold into the marshmallow mixture.

Pour into the tart shell and chill for two hours or until set.

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.

Dust slightly with cocoa.