• Prepare the macaron layers first. Line three baking sheets with parchment paper. On the back of the paper, trace 6-inch (15 cm) circles. Prepare a large pastry bag tipped with a round tip; set aside.
• Place the almond flour, all-purpose flour, coffee and sugar into a food processor. Pulse for 2 to 3 minutes, until a uniform mixture forms. Then sift the dry ingredients into an extra-large mixing bowl using a fine mesh sifter. In a separate mixing bowl, whisk together the egg whites and sugar just until stiff peaks form, about 3 minutes.
• Add the meringue to the dry ingredients and fold thoroughly using a large spatula, pressing the batter against the bowl sides to deflate the meringue. Keep folding and mixing until the batter reaches ribbon stage: ribbons off the spatula smoothly and settles back down on itself within 10 seconds.
• Transfer the batter into prepare pastry bag. Starting from the center of traced circle, pipe the batter onto prepared parchment paper, forming a large, flat disk. If any batter remains, pipe 1-inch macaron disks.
• Tap the baking pan hard against kitchen counter to release any trapped air bubbles. Rest the macarons to set for at least 1 hour, allowing a thin shell to form on the outside. Meanwhile, preheat oven to 320F (160 C) and prepare the coffee custard and salted caramel frosting.

For Coffee Custard:

• Split the vanilla bean in half and add into a sauce pan along with the milk and flour. Whisk the flour in until no clumps remain. Cook the milk over medium-low heat for approximately 5 minutes, until the milk is steaming hot but not boiling. Remove the vanilla bean and scrap out the seeds with a knife and add the seeds to the milk, discarding the pod.
• Meanwhile, in a separate mixing bowl, combine the egg yolks and sugar. Whisk vigorously for 2 to 3 minutes until the mixture is thick and pale and ribbons off of the whisk. In a small ramekin, combine the cornstarch and water to create a slurry. Add the slurry to the egg yolk mixture and whisk for 30 seconds.
• Once the milk is hot, slowly temper the milk into the egg yolks. Add a little at a time and whisk well after each addition. Once the two mixtures have been combined, pour the custard base back into the saucepan. Cook the mixture over medium-low heat for 5 to 7 minutes, stirring constantly with a rubber spatula. Custard is ready when it holds its shape on the back of the spatula. Avoid cooking over high heat; this can cause the custard to separate.
• Remove the custard from heat and add the 1/2 cup butter, coffee and vanilla extract. Whisk the butter into the hot custard until it's completely melted. Cover the custard with plastic wrap touching the surface, to prevent a film from forming on top. Refrigerate the custard until completely cooled.

For Salted Caramel Buttercream:

• Place softened, unsalted butter into mixing bowl. Whisk on high speed for about 5 minutes, until butter is light and fluffy. Scrape down sides of mixing bowl often to get all the butter mixed well. Add the dulce de leche (cooked sweetened condensed milk). If you cannot find dulce de leche, use a similar caramel sauce. If necessary, increase the amount of confectioner's sugar to thicken the frosting.
• Add the vanilla and salt. Start by adding 1/4 teaspoon salt, then add more if needed. Begin adding the confectioner's sugar 1 cup at a time, scraping down sides of mixing bowl often to get everything incorporated. Make sure to sift confectioner's sugar before adding to avoid any clumps. After adding all sugar, whisk on high speed for 2 to 3 minutes to get frosting very fluffy. Transfer the buttercream into a large pastry bag tipped with a large star tip; I used tip #8ft.

Assembling The Cake:

• Once macarons are rested, bake one sheet at a time in the preheated for 18 to 20 minutes. Remove the macarons from the oven and allow them to cool on the parchment paper. Once cooled, gently turn the large macaron upside down and peel away the parchment paper gently.
• To assemble the cake, first pipe dollops of salted caramel buttercream along the edges and add a few dollops into the center; this will help hold up the cake. Fill the center with a generous amount of coffee custard, then top with the next layer. For the top, cover the enter surface with buttercream, then garnish with smaller macarons. For the regular sized macarons, fill them with the salted caramel frosting.
• For best results, allow the cake to chill in the refrigerator overnight; this will make it much easier to cut! Once chilled, use a sharp, serrated knife to slice the cake.