Preheat oven to 350°F (175°C) and grease and flour two 9-inch round cake pans. Use parchment paper rounds in the bottoms of the pans for easy removal.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In a separate bowl, whisk together the buttermilk, oil, eggs, and vanilla extract until smooth.

Gradually add the wet ingredients to the dry ingredients, mixing on low speed until just combined.

Slowly pour in the boiling water while mixing on low speed. The batter will be thin—this is what creates a moist cake.

Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted in the center comes out clean.

Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

In a medium saucepan, melt the sugar over medium heat, stirring constantly with a heat-resistant spatula until it turns amber in color.

Carefully remove from heat and stir in the butter until melted and incorporated smoothly.

Slowly pour in the heavy cream while stirring constantly, allowing the mixture to become smooth and creamy. Stir in sea salt, tasting until you reach your desired saltiness.

In another saucepan, combine the butter, sugar, and water. Cook over medium heat, stirring consistently until the butter melts and the sugar dissolves.

Insert a candy thermometer into the mixture. Continue to cook until it reaches 300°F (149°C), the hard-crack stage.

Remove from the heat and stir in vanilla extract and chopped nuts (if using). Pour the toffee onto a parchment-lined baking sheet, spreading it into a thin layer. Let it cool completely before breaking it into small pieces.

Heat the heavy cream in a saucepan over medium heat until it just begins to simmer.

Remove from heat and pour over chopped chocolate in a heatproof bowl. Let it sit for about a minute until the chocolate melts, then stir until smooth and glossy.

If your cake layers are uneven, level them using a serrated knife. Place one cake layer on a serving plate or cake stand.

Generously drizzle half of the salted caramel sauce over the first cake layer, followed by half of the toffee pieces.

Carefully place the second cake layer on top, then pour the chocolate ganache over it, letting it drip down the sides.

Sprinkle remaining toffee pieces over the top, and drizzle any leftover caramel sauce for an extra touch of indulgence.

Refrigerate the cake for at least 30 minutes to allow the ganache to set. Slice and serve! Ensure the cake is served at room temperature for the best experience.