Preheat the oven to 350F. Butter a 10- to 12-cup Bundt pan and dust with flour or cocoa powder.
In a medium bowl, whisk together the hot water and cocoa powder until well combined. Let cool for 10 minutes
In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt.
To the cocoa mixture, add the melted butter sour cream, oil, eggs, and vanilla. Whisk until well combined. Pour into the flour mixture and stir together just until no streaks of flour remain. Pour into the prepared Bundt pan.
Bake for 55 minutes or until a wooden pick inserted near the center comes out with a few moist crumbs. Let cool in the pan for 20 minutes. Carefully invert onto a wire rack and cool completely.
For the Ganache Topping:
Place the chocolate in a heatproof bowl
Microwave the cream until boiling, about 30 seconds on high. Immediately pour over the chocolate. Drop in the butter but do not stir. Let stand for 5 to 10 minutes.
Stir until the chocolate and butter are melted and the mixture is smooth and silky.
Place the cake on a serving plate and pour the ganache over the top. Store the cake, covered, at room temperature for up to 5 days.
Do not skip blooming the cocoa powder in hot water. Blooming the cocoa gives the chocolate bundt cake a richer chocolate flavor.
As we are working with many eggs, I recommend cracking them into a bowl instead of directly into the batter. This way, you decrease the odds of accidentally getting eggshells in the batter.
Even if you have a non-stick pan, I recommend still dusting the pan. Doing so helps the cake slide out smoothly and lowers the risk of the tops of the cake being stuck and ripping when turned out.
Always check that the baking soda is fresh. Expired baking soda will cause the cake not to rise properly.
Not sure what size bundt pan you have? Use a liquid measuring cup and pour water one cup at a time into the pan, keeping tally as you go, and see how many cups the pan fits.
There’s no need to use a mixer to make the cake batter. You only want to mix the batter until everything is just combined. Overmixing the batter will make the cake dry and tough.
If you are not using a scale to measure the flour, be sure to fluff the flour with a spoon and then sprinkle it into the measuring cups before leveling it off with a knife. While a scale is preferred, fluffing the flour is the best way to measure flour without overpacking the cup.
Use a small spatula or spoon to force the batter into the crevices of the bundt pan. Lightly tap it on the counter after filling to remove air bubbles at the base, which could mar the surface of your cake.