To make the cake, heat oven to 350°F. Grease a square 8x8 baking dish or one 8 inch cake pan.
Stir together flour, sugar, cocoa, baking powder, baking soda and salt in large bowl. Add eggs, buttermilk, oil and vanilla. Beat on medium speed of mixer for two minutes. Beat in the hot coffee. Evenly spread the batter into the prepared baking dish and bake 25 to 30 minutes or until the cake no longer jiggles in the center. Remove and allow to cool 5 minutes. Then poke the cake all over with a fork and evenly pour the Kahlua all over the cake. Cover the baking dish and place in the fridge.
To make the chocolate mousse, in a microwave safe bowl add the chocolate, butter and 2 tablespoons water. Microwave on 30 second intervals, stirring after each, until melted and smooth. Set aside to cool. In a small sauce pan whisk together the egg yolks, 2 tablespoons water and sugar. Place on the stove over medium heat and stir constantly until the sauce thickens slightly and just coats the back of a spoon, about 2-3 minutes. Stir the melted chocolate into egg mixture until completely smooth. Place in the fridge while you whip the cream.
Place 1 1/2 cups of the cream in the bowl of a stand mixer and whip cream until stiff peaks form. Make sure not to over-beat, cream will then become lumpy and butter-like. Now add half the whipped cream to the cooled chocolate mixture. Gently fold the cream into the chocolate until no more streaks remain. Add the remaining cream and gently fold into the mousse. Place in the fridge.
Now add the remaining cream to a clean bowl and whip just as you did earlier. Once whipped add 1 tablespoon powdered sugar and whip until combined. Place in the fridge.
Make the chocolate coffee caramel, whisk together heavy cream and vanilla in a small bowl. Set aside. Combine the sugar and water in a small saucepan over medium-high heat. Bring the mixture to a boil. Once it begins to boil watch closely and whisk until the mixture starts to smell like caramel and turn a golden color, and then to a light brown. This will take about 5-6 minutes. When you see that it has started to turn brown, slowly pour in the cream mixture, whisking constantly and quickly. The sugar will harden around the whisk, don’t worry, it will melt. Just keep whisking! Continue to heat over medium-high until mixture has thickened and is a light caramel color, about 5 minutes more (don't let it get too thick, it will thicken as it sits). Remove from the heat and stir in the, coffee, chocolate chips and vanilla.
To assemble the cakes line up 6 glasses or glass jars. Drizzle the bottom of each jar with 1 tablespoon of the coffee caramel, then crumble in a layer of the cake. Drizzle that with another tablespoon or so of caramel, then add a layer of mouse and a layer of whipped cream. Repeat the layers 2 more times finishing with a layer of whipped cream. Then add crumbled cake to the top of the whipped cream and drizzle with more caramel sauce. Refrigerate 2 hours or until ready to serve.