Adjust the oven rack to the middle position and then preheat oven to 325 degrees.
Spray two 9-inch round cake pans, flour pans, then place parchment paper in the bottom.
To get the perfect parchment paper size, lay your baking pan on a piece of parchment and draw around the base with a pencil.
Cut the circle out, crumble it up, then uncrumple it and lay it in the base of the pan.
Crumbling the piece of parchment helps to hold it in place and not roll up on you.
Do this for both pans and set aside when done.
In a large bowl, sift together 1 1/2 cups flour and 3/4 cup cocoa together.
To the flour mixture, whisk in 1 1/2 cups sugar, 1 1/4 tsp baking soda, 3/4 tsp baking powder, and 3/4 tsp salt and then set aside.
In the bowl of a KitchenAid mixer, whisk 3/4 cup buttermilk, 1/2 cup water, 1/2 cup oil, 2 eggs, and 1 tsp vanilla until combined.
Whisk flour mixture into buttermilk mixture until a smooth batter forms.
Divide the batter evenly between the two cake pans, smooth the tops with a spatula, and then tap the pans on the countertop three times to loosen any air bubbles.
Bake for 22-28 minutes or until a toothpick inserted comes out clean.
Rotate and switch the pans in the oven halfway through baking.
Then remove cake pans from the oven and let them rest to cool, in their pans, on a rack for 15 minutes.
Remove cakes from the pans and then carefully peel off the parchment from the bottom of each cake, discarding the pieces.
Let the cakes cool completely, around 2 hours.
Directions for the Caramel Filling
To begin, lightly grease an 8-inch square baking pan and then set aside.
Combine sugar, corn syrup, and water in a medium saucepan, then bring the ingredients to a boil over medium-high heat and cook without stirring for 8-10 minutes.
Reduce heat and continue to cook, swirling saucepan occasionally, until it turns a dark amber color, 2-5 minutes. The temperature should reach 375-380 degrees.
Then, take the saucepan off heat and add heavy cream, butter, vanilla, and salt. Additionally, this mixture will bubble and steam.
Return saucepan to medium heat and cook, stirring frequently, until the caramel becomes smooth and reaches a temperature of 240-245 degrees; this can take 3-5 minutes.
Carefully transfer caramel to the prepared pan and then set aside to cool, 20-30 minutes.
Directions for the Chocolate Frosting
First, in the bowl of a Cuisinart Food Processor, process the 16 Tbsp butter, 3/4 cup sugar, 1/2 cup cocoa and a pinch of salt with the blade attachment, for 30 seconds until smooth.
Secondly, add 1/2 cup corn syrup and 3/4 tsp vanilla, processing an additional 5-10 seconds.
Additionally, remember to scrape down the sides of the bowl with a spatula as needed.
Add the chocolate and process until creamy, around 10-15 seconds.
Finishing the Chocolate-Caramel Layer Cake
Use a long and serrated knife, score the outside of each cake in a horizontal line around the entire Chocolate Cake, then cut each cake into two pieces.
Using a rubber spatula or large spoon, scoop 1/3 of the caramel onto the tops of three cake layers, then use an offset spatula to spread the mixture over the surfaces leaving a 1/2-inch boarder around each edge.
Line the edges of a cake platter with 3-4 pieces of parchment paper or wax paper to help keep your platter clean of frosting while you decorate. These will be removed later.
Begin to assemble the Chocolate Cake by placing 1 piece of caramel covered cake layers on the cake platter.
Then, top the first layer with the second and third pieces of caramel covered cake layers.
Lastly, top the cake with the final cake layer that has no caramel.
Use a spatula to place dollops of frosting onto the top of the cake.
Finally, use an offset spatula to spread the frosting evenly on the top and edges of the cake, making sure to carefully keep the crumb of the cakes intact.
Carefully remove pieces of parchment or wax paper, discarding them when done.
Let the cake rest for 1 hour before cutting.
Finally, sprinkle with coarse sea salt if using and enjoy!