Prepare the hazelnut meringue layers first; these can be prepared up to 3 days ahead of time. Preheat the oven to 230F/110C. On the back of a large sheet of parchment paper, trace two, 7-inch circles for; one for each layer. Also, prepare a large pastry bag tipped with a round or star tip. Into a super clean mixing bowl, add the egg whites, cream of tartar and granulated sugar. Whisk on low-medium speed for 2 to 3 minutes, until sugar crystals are dissolved, then increase speed to high and whisk for 6 to 8 minutes, until stiff peaks form. Meringue should be thick, glossy and hold its shape very well. Next, fold in the chopped hazelnut and hazelnut extract.
Transfer the meringue into the prepare pastry bag and pipe two even layers onto the prepare paper, about 1-inch thick. Pipe dollops with any remaining meringue. Bake the meringue in the preheated oven for 3 hours; when done, the meringue will sound hollow when tapped on top. Turn the oven off and leave the oven door closed and allow the meringue to cool completely; overnight or for at least 3 to 4 hours.
Next, prepare the coffee cake layers. Preheat the oven to 350F/177C and prepare three, 8-inch (20-cm) cake pans - line the bottoms with parchment paper and spray the sides with a baking spray. Place the softened butter and sugar into a mixer bowl and whisk for 3 to 4 minutes, scraping down the sides of the mixing bowl often, until the butter is light and fluffy. Next, add the eggs and vanilla and whisk again for 3 to 4 minutes, until smooth. Heat the milk in a the microwave for about 1 minute, then add the instant coffee. Stir until the coffee is dissolved, then pour the milk into the cake batter but do not mix.
In a separate bowl, combine the dry ingredients: sifted flour, baking powder and salt. Add the flour to the cake batter and stir just until the flour is incorporated; do not over-mix. Divide the cake batter evenly between the 3 pans and level the batter with an off-set spatula. Bake the layers in the preheated oven for 28 to 30 minutes, or until a toothpick inserted into the center comes out clean. Remove the cakes from the oven and allow them to cool completely on a wire rack. Once cooled completely, use a long serrated knife to level off the tops.
For the Caramel French Buttercream: place the egg yolks into a stand mixer bowl and whisk on medium-high speed for a 3 to 4 minutes until thick and pale. Meanwhile, combine the water and sugar in small saucepan. Heat the sugar, not stirring, over medium-high heat until the temperature reaches 238F/115C, about 5 minutes. Keep the mixer running on medium speed and pour the hot syrup in a slow and steady stream into the egg yolks.
Once all the sugar syrup has been added, turn the mixer up to high speed and whisk for 5 to 7 minutes, until mixture is thick and pale in color. It also needs to be just lukewarm to touch. Begin adding the butter, 1 to 2 tablespoons at a time, mixing for about 15 to 20 seconds after each addition. Scrape down the sides of the mixing bowl often to get all the butter mixed in well. The buttercream will start to thicken as more butter is added. Once all the butter is added, add the dulce de leche, salt and coffee extract. Continue whisking on high speed for 2 to 3 minutes and scrape down the sides of the bowl often to get a uniform buttercream.
To assemble the cake, first soak the cake layers generously with the Kahlua liqueur, using a dispenser bottle or a pastry brush. Spread a generous amount of buttercream between each layer. Layer the cake alternating with the coffee cake layers and meringue layers. The meringue layers will be slightly smaller than the cake layers; fill the space with buttercream. Use flat and off-set spatulas to frost the top and sides of the cake. When finished frosting the cake, allow it to chill in the refrigerator for 30 minutes before adding the chocolate ganache.
For the chocolate ganache, combine the heavy cream and chocolate in a microwave-safe measuring cup. Heat the mixture for about 30 seconds, then stir and heat again until the chocolate is melted. Stir until the ganache is smooth and silky. If ganache is too thick, add more cream; if it's too thin, add more chocolate chips. You can also add 1 tsp shortening to make the ganache shinier. Allow the ganache to cool for at least 15 to 20 minutes, then pour over the edges and over the top of the cake and use an offset spatula to spread the chocolate evenly.
Garnish the top of the cake with caramel coffee macarons, chopped hazelnuts, chocolate covered espresso beans and dollops of the remaining buttercream.