Step 1

Position a rack in the center of the oven and preheat the oven to 350°F. Lightly butter the bottom of two 9-inch round cake pans. Line the bottoms of the pans with parchment paper, and then butter and flour the parchment and the sides of the pans. (Alternatively, replace the butter and flour with nonstick cooking spray with flour, such as Baker’s Joy or Pam with Flour.)

Step 2

In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, cocoa powder, salt, baking powder, and baking soda. Mix on low speed for 30 seconds to combine. Add the eggs, oil, sour cream, and vanilla and mix on low speed until combined.

Step 3

Increase the speed to medium and beat for 2 minutes. Then, reduce the speed to low and gradually pour in the hot water (be careful to pour very slowly so it doesn’t splash). The batter will be soupy. Stop the mixer and scrape down the sides and bottom of the bowl; mix again until evenly combined.

Step 4

Divide the batter evenly between the prepared cake pans. Bake for about 35 minutes, or until a toothpick inserted into the center comes out clean. While the cake is baking, prepare the frosting.

Step 5

Cool the cake layers in the pans on a rack for 10 minutes. Invert the cakes onto a wire rack, remove the pans and cool completely. When the cakes have cooled, place one cake layer on a serving plate. Using an icing spatula or butter knife, spread about ¾ cup of the frosting over the first layer. Top with the second layer, then spread the remaining frosting over the top and sides of the cake, swirling decoratively. The cake is best enjoyed fresh on the day it is made, but it will keep for 2 to 3 days stored in a cake dome at room temperature.

Step 6

Place the chocolate in a microwave-safe bowl and cook in the microwave in 20-second intervals, stirring in between, until it's about three-quarters of the way melted. Stir, allowing the residual heat in the bowl to melt the remaining chocolate completely. (If necessary, place the chocolate back in microwave for a few seconds.) Set aside to cool.

Step 7

In a food processor, process the butter, confectioners’ sugar, cocoa powder and salt until smooth, about 30 seconds, scraping down the sides of the bowl as needed. Add the corn syrup and vanilla and process until just combined, 5 to 10 seconds. Scrape down the sides of the bowl, then add the lukewarm melted chocolate and pulse until smooth and creamy, 10 to 15 seconds. Do not overmix.

Step 8

The frosting can be used immediately or held at room temperature for 3 to 4 hours. It may lose its shine as it sits—to bring the shine back, run a metal spoon under hot water, then wipe dry with a towel; stir the frosting with the hot spoon and it should shine right up.