Preheat your oven to 350 degrees Fahrenheit. This step is crucial as it ensures your cake bakes evenly. While the oven warms up, grab a 9 by 9 or 8 by 8-inch baking dish. Spray it with non-stick cooking spray and dust it with a little cocoa powder, shaking out any excess. This will help the cake release easily once baked.
In a large mixing bowl, whisk together the flour, sugar, 6 tablespoons of cocoa powder, baking soda, and salt. This dry mixture is the base of your cake. Whisking helps to aerate the ingredients, ensuring even distribution of the baking soda and salt.
Next, add the buttermilk, vegetable oil, vanilla, and eggs to the bowl. Using a whisk, mix these wet ingredients into the dry ingredients until they are very well blended. This should take about one minute, and you should expect a smooth batter.
Once your batter is ready, pour it into the prepared baking dish. Ensure it's spread evenly; you can give the dish a gentle shake to level it out. The batter should fill the dish about halfway up.
Place the dish in your preheated oven and bake. You’ll want to check the cake after about 27 to 35 minutes by inserting a toothpick into the center. If it comes out clean, your cake is ready!
After baking, allow the cake to cool completely on a wire rack. This step is vital to ensure that the ganache sets perfectly on top without melting.
While the cake cools, let’s prepare the ganache. Start by placing the finely chopped semi-sweet chocolate in a medium heat-proof mixing bowl. This will be the base of our delicious frosting.
In a small saucepan over medium heat, heat the heavy cream until it just starts to simmer. Be careful not to let it boil. As soon as you see steam, remove it from the heat.
Pour the hot cream over the chopped chocolate. Allow it to sit for a minute before gently stirring with a rubber spatula until melted and smooth. The heat from the cream will melt the chocolate beautifully.
Let the ganache cool at room temperature for about 30 minutes until it thickens slightly. Once the cake is cool and the ganache is at the right consistency, pour it over the cooled cake, letting it drip down the sides. Finally, slice and serve your delicious creation!