Preheat oven to 320 degrees F (10 degrees C) and grease a 9-inch baking pan.
Add Powdered Confectioners Sugar (1 1/2 cup) to Thick Yogurt (1 1/2 cup), mix well.
Add Baking Powder (1 1/2 teaspoon), Baking Soda (1 teaspoon) and Salt (1 pinch). Mix, then let it rest for 5 minutes until you see a lot of air bubbles.
Add Vanilla Extract (1 teaspoon) and the Vegetable Oil (1/4 cup) and mix well.
Sift in All-Purpose Flour (2 1/2 cup) and Unsweetened Cocoa Powder (1/2 cup). Mix to form a smooth batter. Do not over mix.
Pour into prepared baking pan. Bake for 30 minutes or until the cake is fully cooked through. Let it cool completely.
For the caramel dark chocolate truffle frosting, add Granulated Sugar (1/2 cup) and Water (2 tablespoon) to a pan and let it come to a boil on lowest heat. Do not stir or the sugar will crystallize.
Once the sugar turns a light brown color, add two thirds of the Butter (2 tablespoon) and Cream (1/2 cup). Stir.
Pour caramel onto Dark Chocolate (3/4 cup) and let it sit for 5 minutes.
Stir well to combine. It should form a smooth ganache like sauce. Let it cool completely and refrigerate to get a thick truffle frosting.
For the chocolate mousse filling put Marshmallow (1/3 cup), Unsalted Butter (1 tablespoon), Dark Chocolate Chips (1/2 cup), and Water (1 tablespoon) in a heavy-based saucepan over medium heat.
Once everything is smooth and melted, remove from heat and let it cool a bit. Stir frequently to keep skin from forming.
Whip Heavy Cream (1/2 cup) and Vanilla Extract (1 teaspoon) in a cold bowl until the cream forms stiff peaks.
Add chocolate mixture into the whipped cream and gently fold until you have a smooth, cohesive mixture. Let it chill completely until thick and mousse like.
To form the cake, cut into 2 or 3 layers. Poke holes all over the cake with a fork and brush with Sugar Syrup (to taste) to keep it moist.
Add a dollop of the chocolate mousse in the center. Spread it leaving about an inch on all sides.
Add Cookies (1/4 cup) and Milk Chocolate Chips (1/4 cup) on top of the mousse.
Close with the next layer of the cake. Again poke holes, and brush with sugar syrup. And cover with the thick truffle frosting. Use a cake knife to smooth it out as much as possible.
Design the top as you please with the frosting and cookie!