Preheat your oven to 350°F (175°C). Grease and flour your 9-inch round cake pans, ensuring they are ready to release the cake after baking.
In a large mixing bowl, whisk together the cocoa powder and boiling hot coffee until smooth. Let it cool slightly while you work on the dry ingredients.
In another bowl, combine the flour, sugar, baking powder, baking soda, and salt. Mix well to ensure even distribution of the leavening agents.
In a separate bowl, beat the eggs until frothy. Add the vegetable oil, Greek yogurt, vanilla extract, and buttermilk. Stir well to combine all the wet ingredients.
Now, pour the cocoa-coffee mixture into the wet ingredients and mix until well combined. Gradually add the dry ingredient mixture to the wet mixture, stirring gently until just combined. Be careful not to overmix; a few lumps are perfectly fine.
Divide the batter evenly between the prepared cake pans. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Once baked, remove the pans from the oven and let the cakes cool in the pans for about 10 minutes before transferring them to a cooling rack to cool completely.
While the cakes are cooling, prepare the chocolate filling. In a saucepan, whisk together the milk, sugar, cornstarch, and vanilla extract. Heat over medium heat, stirring constantly until the mixture begins to thicken.
Once thickened, remove from heat and whisk in the chopped chocolate until smooth. Allow this mixture to cool slightly, then whisk in the egg until fully incorporated.
Once the cakes are completely cool, place one layer on a serving plate. Spread the chocolate filling evenly over the top. Carefully place the second cake layer on top of the filling.
To make the ganache, heat the heavy cream in a saucepan until it just begins to simmer. Pour it over the chopped semisweet chocolate in a heatproof bowl. Let it sit for a few minutes, then whisk until smooth and shiny. Add a pinch of salt to enhance the flavor.
Pour the ganache over the top of the assembled cake, allowing it to drip down the sides for a beautiful finish. Use a spatula to spread the ganache evenly if desired. Let the ganache set for a bit before slicing.
