Grease three 8-inch round cake pans, line bottoms with parchment, and grease again. Position a rack in the lower third of the oven and preheat to 350°F (175°C). Set aside while you prepare the cake batter.

In a large mixing bowl, sift together the flour, Dutch-process cocoa powder, baking powder, and fine sea salt. Whisk thoroughly to combine and aerate, ensuring the cocoa is completely distributed for an even crumb.

In a separate bowl, whisk together granulated sugar, neutral oil, eggs, vanilla extract, and sour cream until smooth and glossy, with no streaks of egg or lumps of cream. This mixture should look thick and cohesive.

Add the wet mixture to the sifted dry ingredients and mix with a paddle or large spatula until just combined, do not overmix. Pour in the hot water last, stirring gently until the batter is fully smooth and glossy.

Divide the batter evenly among the three prepared pans. Tap each pan gently to release large air bubbles, then bake for 25-29 minutes or until a toothpick inserted at the center comes out with a few moist crumbs, not wet batter.

Let cakes cool in their pans on racks for 10-12 minutes. Run a thin offset spatula around the edges, invert onto racks, and peel off the parchment. Once completely cool, use a long serrated knife to level each layer if necessary.

Beat 170g unsalted butter with a stand mixer until very light and fluffy, 3-4 minutes. Gradually add sifted powdered sugar and 1/4 teaspoon sea salt, mixing slowly at first to avoid a sugar cloud, then increase speed. Add vanilla and 60ml cream, beating on medium-high until thick, creamy and smooth.

Place the first cake layer on a turntable, spread about 3/4 cup of buttercream over the surface, then repeat with the second layer. Top with the third layer. Spread a thin crumb coat over the entire cake, scraping excess back into the bowl. Chill for at least 30 minutes.

Apply the remaining buttercream evenly over the chilled cake. Use an offset spatula and bench scraper to smooth the sides and top, spinning the cake turntable for clean lines and sharp edges. Chill again for at least 45 minutes before dripping.

Place chopped bittersweet chocolate in a heatproof bowl. In a small saucepan, heat 180ml heavy cream until steaming (not boiling), then pour over the chocolate. Let sit 2 minutes, then whisk until shiny and completely smooth. Cool to room temperature; ganache should be thick but pourable.

Transfer ganache to a squeeze bottle or use a spoon to carefully drip the ganache along the edges of the chilled cake, allowing it to run down naturally. Fill the center with more ganache and spread gently for a glossy finish. Let set 20 minutes before slicing or adding further decorations.