Step 1

Preheat the oven to 350F. Butter a 10- to 12-cup Bundt pan and dust with flour or cocoa powder.

Step 2

In a medium bowl, whisk together the hot water and cocoa powder until well combined. Let cool for 10 minutes

Step 3

In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt.

Step 4

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.

Step 5

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.

Step 6

Place the chocolate in a heatproof bowl

Step 7

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.

Step 8

Stir until the chocolate and butter are melted and the mixture is smooth and silky.

Step 9

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.