For the cake: Preheat the oven to 350 degrees F. Grease three 8-inch round cake pans and then flour the insides of the pans, tapping out excess.
In the bowl of a stand mixer fitted with a paddle attachment (or in a large mixing bowl if you're using a hand mixer), sift together the flour, sugar, cocoa powder, baking soda, baking powder and salt. In a medium bowl, whisk together the eggs, buttermilk, coffee, oil and vanilla.
Add the wet ingredients to the dry ingredients, and mix for 2 minutes on medium speed. Scrape the sides and bottom of the bowl; mix for an additional 20 seconds--the batter will be very thin. Divide the batter evenly among the prepared pans.
Bake for 20 minutes. Rotate the pans and continue baking until a toothpick inserted into the center of one of the cakes comes out almost clean, with just a few moist crumbs, about 12 minutes more. Cool the cakes in the pans on a wire racks for 20 minutes, then carefully turn out onto cooling racks to cool completely.
For the nougat filling: Melt the butter in a medium saucepan over medium heat. Add the sugar and evaporated milk, stirring until dissolved. Bring to a boil, then reduce the heat to low and cook for 5 minutes, stirring occasionally. Remove the pan from the heat and add the marshmallow fluff, peanut butter and vanilla extract, stirring until completely smooth. Fold in the peanuts. Let the nougat mixture cool to room temperature, stirring occasionally, before using in the cake--you can do this by leaving it at room temperature, or you can place it into the refrigerator to speed up the process. (Makes 2 cups.)
For the salted-caramel buttercream frosting: In a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed for 5 minutes. Pour in the salted caramel sauce, salt and vanilla, and beat until combined. Reduce the speed to medium-low, and slowly add the powdered sugar, a little at a time, until fully incorporated. Increase the speed to medium-high and beat until light and fluffy, an additional 2 to 3 minutes. (Makes 2 cups.)
To assemble: Level off the tops of the cakes if they baked up uneven or domed. Place one cake layer on a serving plate. Cover with half of the nougat filling. Top with a second cake layer, and cover with the remaining filling. Place the final cake layer, face-down, on top.
Frost the cake with the salted-caramel frosting, finishing it as smoothly as possible. Drizzle with caramel if desired.
Refrigerate for at least 1 hour, until the frosting is set.
Recipe courtesy of Meghan Smith