Preheat your oven to 350°F and prepare three 8-inch cake pans by greasing and lining them with parchment paper. If you're working with fewer pans, bake the layers in separate batches.

In a large mixing bowl, sift together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt.

In the bowl of a stand mixer, combine the sugar, milk, oil, eggs, and vanilla extract using the paddle attachment. Mix until fully blended.

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

While the mixer is running on low, carefully pour in the boiling water, ensuring the batter is smooth. Scrape the sides of the bowl as needed. Note that the batter will have a thin consistency.

Evenly divide the batter between the prepared cake pans and bake for 25-30 minutes, or until a toothpick inserted into the center of each cake comes out clean.

Allow the cakes to cool in their pans for 10 minutes before transferring them to wire racks to cool completely.

In the bowl of a stand mixer fitted with the whisk attachment, beat the softened butter until it becomes smooth and creamy.

Gradually add the powdered sugar, mixing on low after each addition. Once all the sugar is incorporated, increase the mixer speed to medium and beat for an additional 3 minutes until the frosting becomes light and fluffy.

Add the caramel sauce, vanilla extract, and salt, and beat for another 2 minutes on medium speed. The frosting should be smooth and spreadable. If it feels too stiff, you can add a small amount of milk to loosen it up. If it's too soft, mix in a little more powdered sugar.

Place the dark chocolate chips in a heatproof bowl.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Immediately pour the hot cream over the chocolate chips. Let it sit for about 2 minutes before whisking it into a smooth and glossy ganache.

If necessary, trim the tops of the cake layers to ensure they are flat and even. Place one layer of cake on your serving plate or cake turntable.

Spread a layer of the caramel buttercream frosting over the top of the cake, leaving a small border around the edges.

Place the second cake layer on top and repeat the frosting process. Add the final cake layer, and apply a thin crumb coat of frosting over the entire cake. Chill the cake in the refrigerator for at least 30 minutes to set the crumb coat.

Once chilled, apply a second, thicker layer of buttercream over the top and sides of the cake.

Pour the prepared ganache over the top of the cake, allowing it to flow gently to the edges. Use a spoon to guide some ganache over the sides in evenly spaced drips.

For extra decoration, drizzle additional caramel sauce over the top of the ganache, and sprinkle a pinch of flaky sea salt if desired.

Using a piping bag fitted with a Wilton 2D nozzle, pipe swirls of the remaining caramel buttercream on top of the cake.

Place the cake back in the refrigerator for another 30-45 minutes to allow the ganache to set before slicing and serving.