Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Prepare the Dry Ingredients: In a large bowl, sift together the flour, baking powder, baking soda, cocoa powder, and salt.
Melt the Chocolate: Place the chopped dark chocolate in a heatproof bowl and melt it using a microwave or over a double boiler. Stir to combine once melted, and let it cool slightly.
Mix the Wet Ingredients: In another bowl, beat together the granulated sugar, brown sugar, eggs, buttermilk, vegetable oil, and vanilla extract until smooth.
Combine the Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Stir in the melted chocolate.
Add Sour Cream and Hot Liquid: Mix in the sour cream, then gradually add the hot water (or coffee) while stirring. The batter will be thin—this is normal.
Bake the Cake: Pour the batter evenly into the prepared pans. Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
Prepare the Ganache: In a small saucepan, heat the heavy cream until hot but not boiling. Remove from heat and add the chopped semisweet chocolate, butter, and corn syrup. Stir until the ganache is smooth and glossy.
Assemble the Cake: Once the cakes are completely cool, spread a layer of ganache on top of one cake layer. Place the second layer on top, and pour the remaining ganache over the entire cake, letting it drip down the sides.
Serve: Allow the ganache to set for a few minutes before slicing and serving. Enjoy!