Preheat your oven to 350°F and position your rack in the center. Line the bottom of two 9-inch round cake pans with parchment paper, then grease the entire inside of each pan with butter or cooking spray.

Sift together the flour, cocoa powder, baking soda, baking powder, and salt into a large bowl. Set this mixture aside.

In another large bowl, beat the sugar and eggs together with an electric mixer on medium speed for about 3 minutes until the mixture turns pale and fluffy. Scrape down the bowl halfway through.

With the mixer running on low speed, slowly drizzle in the vegetable oil while continuing to beat. Once the oil is fully incorporated, add the vanilla extract and mix for another 30 seconds.

Reduce the mixer speed to low and add one-third of the dry mixture, then one-third of the buttermilk, alternating until everything is incorporated. Start and end with dry ingredients and mix only until you don't see dry streaks.

Carefully stir in the hot water by hand using a wooden spoon or spatula, mixing just until combined. The batter will look thin and pourable.

Divide the batter evenly between your prepared pans and smooth the tops with a spatula. Bake for 28 to 32 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs.

Let the cakes cool in their pans for 15 minutes, then turn them out onto a wire rack to cool completely (about 45 minutes to 1 hour).

While the cakes cool, warm the caramel sauce in a saucepan over low heat. Add the heavy cream, butter, and sea salt, whisking until smooth. Let cool for 5 minutes before using.

To make the frosting, beat the softened butter for 2 minutes until creamy. Sift the cocoa powder and powdered sugar together, then add to the butter in three additions, beating on medium speed. Add the heavy cream, vanilla, and salt, then beat on medium-high speed for 3 minutes until fluffy.

Place one cooled cake layer on your serving plate and spread a thin layer of chocolate frosting on top. Drizzle or spread half of the caramel mixture over this frosting layer, leaving a 1/2-inch border around the edges. Top with the second cake layer and press gently.

Spread a thin crumb coat of chocolate frosting over the entire cake. Refrigerate for 15 minutes to set. Apply the final frosting layer generously using an offset spatula.

Chill the cake for at least 2 hours after frosting before serving so the layers set properly and the caramel stays gooey.