Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, or line with parchment paper.
Step 2: In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Step 3: Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes, scraping down the sides of the bowl as needed.
Step 4: Carefully stir in the hot coffee (or hot water). The batter will be thin. Pour evenly into the prepared cake pans.
Step 5: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10-15 minutes, then invert onto a wire rack to cool completely.
Step 6: To make the caramel sauce: In a heavy-bottomed saucepan, combine sugar and water over medium heat. Stir until the sugar dissolves, then stop stirring. Cook until the mixture turns an amber color, swirling the pan occasionally (about 8-10 minutes).
Step 7: Remove from heat and carefully pour in the warm heavy cream (it will bubble vigorously). Stir until smooth. Add butter and sea salt, stirring until melted and combined. Let cool completely to thicken.
Step 8: To make the chocolate buttercream: In a large bowl, beat the softened butter with an electric mixer until light and fluffy (about 2-3 minutes).
Step 9: Gradually add the sifted powdered sugar and cocoa powder, alternating with the heavy cream and vanilla extract. Beat until smooth and creamy, adding a pinch of salt. If too thick, add more cream; if too thin, add more powdered sugar.
Step 10: Once cakes are completely cool, level the tops if necessary. Place one cake layer on your serving plate.
Step 11: Pipe a dam of chocolate buttercream around the perimeter of the cake layer. Fill the center with about ½ cup of caramel sauce and sprinkle generously with toffee bits.
Step 12: Carefully place the second cake layer on top and repeat the filling process.
Step 13: Cover the entire cake with a thin layer of chocolate buttercream (this is the crumb coat). Chill the cake in the refrigerator for 15-20 minutes to set the crumb coat.
Step 14: Apply the remaining chocolate buttercream smoothly over the top and sides of the cake. Drizzle with more caramel sauce and sprinkle with extra toffee bits for decoration.
Step 15: Serve immediately or refrigerate. Allow to come to room temperature for 15-20 minutes before serving for best flavor and texture.