Preheat your oven to 350°F and line your cake pan with parchment paper. While the oven heats, combine the flour, unsweetened cocoa powder, baking powder, baking soda, 3/4 cup sugar, and salt in a large mixing bowl. Whisk these together thoroughly to distribute the leavening agents and cocoa evenly—this ensures your cake will rise uniformly and have consistent chocolate flavor throughout.

Add the room-temperature eggs, sunflower oil, sour cream, and vanilla extract to the dry mixture from Step 1. Stir until the ingredients are just combined and smooth. Then gradually pour in the 1/2 cup hot water while stirring—the hot water will thin the batter and help it bake evenly, creating a moist, tender crumb. The batter should be thin and pourable at this point, which is exactly what you want.

Pour the batter from Step 2 into your prepared pan and bake for 20-25 minutes, until a skewer inserted in the center comes out clean or with just a few moist crumbs. Allow the cake to cool completely in the pan—this is important because it's still delicate when warm and needs structure before you level it. Once cooled, gently remove the cake from the pan, level off the top with a serrated knife if needed, and place it back in the pan for the next steps.

Place the 7 oz of chopped dark chocolate in a heat-safe bowl. Heat 1 cup of heavy whipping cream with 1 tbsp corn syrup in a saucepan until it reaches a bare simmer. Pour this hot cream mixture over the chocolate and let it sit undisturbed for 2 minutes—the residual heat will melt the chocolate gently and evenly. Stir until completely smooth and glossy, then set aside to cool until it reaches a spreadable consistency, about 10-15 minutes. I find that letting the ganache cool slightly before adding the whipped cream prevents the cream from deflating, keeping your filling light and airy.

While the chocolate ganache cools, whip the 1/3 cup heavy whipping cream with 2 tbsp sugar until soft peaks form. Gently fold one-third of this whipped cream into the cooled ganache from Step 4 to lighten it, then carefully fold in the remaining whipped cream until just combined—be gentle to maintain the airy texture. Spoon this truffle mixture evenly over the cooled cake in the pan, smooth the top with an offset spatula, and refrigerate for 2 hours until the filling is completely set.

Place the 4 oz of chopped semisweet dark chocolate in a bowl. Heat the 1/2 cup heavy whipping cream with 1 tsp corn syrup until it simmers, then pour it over the chocolate. Stir until completely smooth and glossy, then let it cool for 15 minutes—this cooling period is crucial because if the glaze is too hot, it will sink into the truffle layer instead of setting on top. Once properly cooled, pour the glaze over the chilled cake and spread it gently and evenly. I like to let the glaze find its own level slightly before smoothing it completely, which gives it a more elegant, natural finish.

Return the cake to the refrigerator until the glaze is completely set, about 1-2 hours. Just before serving, lightly dust the top with cocoa powder for a classic truffle cake presentation. Run a warm knife around the edges before slicing to ensure clean cuts—the residual heat from the knife will help the slices release smoothly from the pan.