Preheat the oven to 350 F (180C). Line a 9x13 pan with parchment paper, with excess strips hanging over the sides so it's easy to lift the cake out once it's done.
In a large bowl, sift together the flour, cocoa, baking powder, baking soda, and salt. Add the brown sugar then whisk to combine.
Make a well in the middle of the dry ingredients, and add the eggs, vanilla, sour cream, coffee, and vegetable oil. Whisk until smooth, but don't overmix.
Pour the cake batter into the prepared pan. Bake for 25-30 minutes or until a toothpick inserted in the center of the pan comes out clean, and cake is springy to the touch.
Cool the cake in the pan for 20 minutes, then lift it out onto a wire rack to continue cooling.
Once the cake is completely cool to the touch, pour the set ganache into the bowl of a stand mixer (or use a hand mixer and the same bowl). With a whisk attachment, beat on medium speed for 1-2 minutes until thick and fluffy.
Spread the ganache over the cake, and decorate with chocolate shaving, chopped candy bars, peanuts, sprinkles, or any other nut of choice. Enjoy! Store any leftovers in the fridge.