Preheat oven to 350 degrees. Spray a 12 - 15 cup non-stick bundt cake with non-stick cooking spray. Dust lightly with flour and cocoa powder (about 1/2 Tbsp of each). Shake out excess.
In a medium mixing bowl whisk together flour, sugar, baking soda and salt. Set aside.
In a large mixing bowl whisk together melted butter, hot water and cocoa powder.
Add flour mixture to butter mixture and using an electric hand mixer blend until well combined.
Add sour cream, eggs and vanilla extract. Mix until well combined.
Pour mixture into prepared bundt pan.
Bake in preheated oven until toothpick inserted into center comes out clean, about 40 - 50 minutes.
Let cool 8 - 10 minutes then run a knife down the sides of cake to ensure they're loose. Invert onto a wire rack to cool at least 30 minutes.
To make the chocolate ganache place chocolate in a medium heat proof mixing bowl.
Heat heavy cream in a small saucepan on the stove until just simmering. Pour hot cream over chocolate (submerge chocolate if needed).
Let mixture rest 4 minutes then stir until melted and smooth.** Pour chocolate mixture over cake. Let cool slightly.
Cut into slices and serve.
Notes
*Salted butter can be used in place of unsalted if that's all you have. Just omit the 1/2 tsp salt from the recipe.
**If chocolate hasn't completely melted you can heat over a double boiler stirring until melted or in the microwave in 20 second intervals on 50% power stirring between intervals.
While the cake is cooling I like to cover with a cake carrier or big bowl while cooling to help soften the exterior.
If you didn't plan ahead it's not critical to use room temperature ingredients. It just improves the rise only slightly.