Preheat your oven to 350°F (175°C) and prepare three 8-inch cake pans by greasing and lining them with parchment paper. In a mixing bowl, whisk together the dry ingredients: all-purpose flour, cocoa powder, baking powder, baking soda, and kosher salt. In a separate jug, combine the wet ingredients: buttermilk, vegetable oil, eggs, and vanilla extract. Gradually mix the wet ingredients into the dry, then slowly stir in boiling water until you achieve a smooth batter.

Divide the thin batter evenly between the prepared cake pans, roughly 600 grams per pan. Bake in the preheated oven for 35-40 minutes. To check if the cakes are done, insert a toothpick into the center; it should come out clean or with a few moist crumbs attached. Once baked, remove the cakes from the oven and let them cool in the pans.

Allow the cakes to cool in their pans for about 10 minutes. After that, carefully transfer them to wire racks to cool completely.

In a mixing bowl, cream together the room temperature butter, light brown sugar, and granulated sugar until fluffy. Gradually add milk to reach your desired consistency, then fold in heat-treated flour and chocolate chips. Divide the cookie dough into discs, wrap them in cling film, and place them in the refrigerator to chill.

In a large bowl, beat the butter until creamy, then add cocoa powder and powdered sugar, mixing until fluffy. Gradually incorporate melted chocolate and heavy cream to achieve a smooth, spreadable consistency.

Once your cakes are completely cool, place one layer on a serving plate. Spread a generous layer of cookie dough filling on top, then place another cake layer over it. Repeat with the remaining layers and top the whole cake with the rich fudge frosting.