Prep your pans and oven. Heat oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment. Lightly dust with cocoa powder to prevent sticking.

Whisk dry ingredients. In a large bowl, whisk flour, granulated sugar, brown sugar, cocoa, hot chocolate mix, baking powder, baking soda, and salt until evenly combined.

Mix wet ingredients (minus coffee). In another bowl, whisk eggs, buttermilk, oil, and vanilla until smooth.

Combine and add hot liquid. Pour the wet mixture into the dry and stir until almost combined. Add the hot coffee and mix just until smooth. The batter will be thin.

Bake. Divide the batter between pans. Bake 25–30 minutes, or until a toothpick comes out with a few moist crumbs. Cool in pans 10 minutes, then turn out onto racks to cool completely.

Make the salted caramel. In a light-colored saucepan, melt sugar over medium heat, stirring as it clumps, then liquefies and turns amber. Add butter and whisk (it will bubble). Slowly pour in cream, whisking until smooth. Remove from heat, stir in sea salt, and cool to room temperature.

Prepare frosting. Beat butter until fluffy. Sift in powdered sugar and cocoa; mix on low, then medium. Add vanilla, a pinch of salt, and the cooled caramel. Beat, adding cream 1 tbsp at a time until spreadable.

Assemble. Place the first cake layer on a stand. Spread a thin layer of frosting, then drizzle 2–3 tbsp caramel over it, staying 1/2 inch from the edge. Top with the second layer.

Frost and finish. Apply a thin crumb coat and chill 15 minutes. Frost generously. Drizzle with more caramel and top with mini marshmallows or chocolate curls. Sprinkle a little flaky salt for a final pop.