Preheat oven to 350°F. Coat 2 (8-inch) round cake pans with cooking spray; line bottoms with parchment paper, and spray parchment with cooking spray. Set aside.

Whisk together flour, sugar, cocoa, instant espresso, baking soda, baking powder, and salt in a large bowl until combined.

Make a well in center, and add eggs, sour cream, oil, buttermilk, and vanilla; whisk until just combined, about 30 seconds.

Whisk in hot water until just incorporated.

Divide batter evenly between prepared pans (about 2 ¼ cups each).

Bake in preheated oven until a wooden pick inserted in center of cakes comes out with moist crumbs, 25 to 30 minutes.

Let cakes cool in pans on a wire rack for 10 minutes. Remove cake layers from pans; cool completely on wire rack, about 1 hour.

Bring cream and sugar to a boil over medium-high, stirring occasionally. Reduce heat to medium-low, and simmer, stirring occasionally, until liquid reduces to 2 cups, about 6 minutes.

Pour sugar mixture into a medium bowl, and add chocolate, butter, vanilla, and salt. Let stand, stirring occasionally, until chocolate and butter are melted and mixture is smooth, about 3 minutes.

Set bowl of Frosting in a large bowl of ice water. Using a hand mixer, beat Frosting on medium speed until thick, glossy, and lightened in color, about 3 minutes, scraping down sides and bottom of bowl occasionally.

To assemble the Cake, place 1 cake layer, top side down, on a plate or cake stand. Top with about 1 ½ cups Frosting, and spread evenly to edges using an offset spatula. Top with second cake layer, top side down. Spread remaining Frosting evenly over top and sides of Cake; decorate as desired.