Preheat oven to 350°F and set the oven rack to the middle position.
Grease a 10-inch tube pan with butter or shortening and sprinkle it with cocoa. Or spray the pan with nonstick spray (the type with flour included).
In a small microwave-safe bowl, melt the chocolate morsels at medium heat for 30 seconds. Stir the chocolate and continue microwaving and stirring at 15-second intervals until melted. Stir until creamy and smooth.
1½ cups semisweet chocolate morsels
In a medium bowl, sift the flour, baking soda, and salt together. Set aside.
2 cups all-purpose flour, 1 teaspoon baking soda, ½ teaspoon kosher salt
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and brown sugar together at medium speed until fluffy and completely incorporated, about 5 minutes.
½ cup unsalted butter, 16 ounces brown sugar
Add the eggs, one at a time, beating just until incorporated after each addition.
3 large eggs
Add the melted chocolate and mix just until well blended.
On low speed, slowly add the flour mixture and the sour cream alternately to the chocolate mixture. After each addition, mix the ingredients just until incorporated. Begin and end with the flour mixture. (The additions are added as follows: ⅓ of the flour mixture, ½ of the sour cream, ⅓ of the flour mixture, ½ of the sour cream, ⅓ of the flour mixture.)
8 ounces sour cream
Very gradually add the hot water and beat on low just until blended.
1 cup water
Add vanilla and stir.
2 teaspoons pure vanilla extract
Spoon the batter evenly into the prepared tube pan.
Bake at 350°F for 55-65 minutes or until a toothpick inserted in the center comes out clean. Transfer the cake to a wire rack and let cool in the pan for 10 minutes. Lift the cake by the center tube and return to the wire rack.
While the cake cools, make the ganache. In a large microwave-safe bowl, heat the cream on medium heat until a few bubbles are visible (just beginning to boil).
¾ cup heavy cream
Add the chocolate morsels, stirring every 15 seconds or until the chocolate melts.
11 ounces semisweet chocolate morsels
Add the butter and stir until fully incorporated.
1 tablespoon unsalted butter
Pour the ganache (see note) over the cake and sprinkle with flaked sea salt.
⅛ teaspoon flake sea salt