Preheat your oven to 350°F. Lightly grease a 9 inch Bundt pan with vegetable oil or butter. If your cakes tend to stick to your Bundt pan, I'd recommend giving it a light dusting with flour as well. Set aside.
In a medium bowl, combine flour, cocoa powder, baking powder, baking soda, and salt. Whisk to combine and set aside.
In a separate large bowl, combine sugar, buttermilk, oil, eggs, and vanilla. Whisk to combine.
Add the dry ingredients to the wet ingredients, and fold together until just combined. The batter will be thick at this stage.
In a small bowl or measuring cup, combine hot water and espresso powder, and mix until the coffee is fully dissolved. Pour the hot coffee into the cake batter. Working relatively quickly, whisk the coffee into the batter until just combined. It should be quite loose now (that's normal!)
Pour the cake batter into your prepared Bundt pan and smoothen into an even layer. Bake the cake in the center of your preheated 350°F oven for 45 to 55 minutes, or until puffed and a skewer inserted into the center of the cake comes out clean.
Cool your cake for 10 to 15 minutes in your cake pan, then turn onto a wire rack to cool completely to room temperature.
Make your ganache: In a medium bowl, combine chocolate and heavy cream and melt in your microwave or over a bain-marie. To melt in the microwave, heat the mixture on high for 30 seconds. Pause to stir, and continue heating in 10 second intervals, stirring after each burst, until fully melted and smooth. To melt over a bain-marie, fill a small pot with about 1 inch of water and bring the water up to a gentle simmer. Place the bowl on top of the pot (making sure the bottom of the bowl doesn't touch the water). Heat, stirring occasionally, until the mixture is melted and smooth.
Let the chocolate ganache cool for about 5 to 10 minutes to set up slightly. Pour your chocolate ganache on top of your cooled chocolate cake. Slice and serve!