1. Prepare Oven and Pan:

• Preheat the oven to 170°C (approximately 340°F).
• Grease your chosen cake pan(s) very well. Consider lining the bottom with parchment paper for easier removal.

2. Whip Eggs and Sugar:

• In a mixer bowl fitted with the whipping attachment, whip the eggs, sugar, salt, and vanilla extract on medium-high speed until the mixture is airy, light, pale, and somewhat foamy. This typically takes several minutes.

3. Add Wet Ingredients:

• Switch to the paddle attachment (if using a stand mixer).
• Add the sour cream and vegetable oil to the egg mixture.
• Mix slowly on medium speed just until they are absorbed into the mixture. Do not overmix.

4. Add Dry Ingredients:

• Sift the all-purpose flour, cocoa powder, and baking soda together directly into the mixer bowl (or sift into a separate bowl first).
• Mix slowly on low speed until the dry ingredients are almost uniform. Some streaks of flour are okay at this point.

5. Add Water and Finish Batter:

• With the mixer on low speed, slowly pour in the lukewarm water.
• Mix slowly at medium speed only until you achieve a completely uniform batter. Be careful not to overmix once the flour is fully incorporated.

6. Pour and Bake:

• Pour the batter into the prepared cake pan(s).
• Bake for 50-60 minutes (time will vary depending on pan size and shape) or until the cake is very swollen, feels steady to the touch (springs back lightly), and a skewer inserted into the center comes out with moist crumbs attached (not wet batter).

7. Cool the Cake:

• Cool the cake completely in the pan(s) on a wire rack at room temperature. Rushing this step can cause the cake to break.

8. Prepare Cake Layers:

• Once completely cool, carefully remove the cake(s) from the pan(s).
• Using a long, serrated bread knife, level the top of the cake(s) if necessary to create a flat surface.
• If baking one large cake, slice it horizontally into two equal layers.

9. Make the Chocolate Ganache:

• Finely chop the dark chocolate and place it in a heatproof bowl.
• Heat the whipping cream in a saucepan until it just begins to simmer (or heat carefully in the microwave). Do not boil vigorously.
• Pour the hot cream over the chopped chocolate. Let it sit undisturbed for 5-10 minutes to allow the chocolate to melt.
• Gently whisk the mixture, starting from the center and working outwards, until the ganache is completely smooth, uniform, and shiny.

10. Cool the Ganache:

• Cool the ganache for 4-5 hours at room temperature, stirring occasionally, until it reaches a thick, spreadable, paste-like consistency (similar to peanut butter).
• To speed up cooling, you can refrigerate the ganache, but you must stir it vigorously every 10-15 minutes to ensure even cooling and prevent it from becoming too hard.

11. Assemble the Cake:

• Place the bottom layer of the cake on a serving plate or cake stand.
• Spoon about one-third of the cooled, thickened ganache onto the cake layer.
• Using an offset spatula or palate knife, spread the ganache into a uniform layer.
• Carefully place the second cake layer on top of the ganache.

12. Frost the Cake:

• Cover the top and sides of the cake with almost all the remaining ganache, using your offset spatula to create a smooth, uniform layer.

13. Decorate:

• Transfer the small amount of remaining ganache into a piping bag fitted with a 1 cm serrated piping tip (or your desired tip).
• Pipe decorative elements, like rosettes or shells, around the top edge or center of the cake.

14. Serve and Store:

• Serve the cake at room temperature for the best texture.
• It’s recommended to remove the cake from the refrigerator about an hour before serving.
• Keep the cake stored in a closed, airtight container. It can last for up to a week (refrigeration may extend this, but bring to room temp before serving).