The Chocolate Cake

• Adjust an oven rack to the middle position and preheat the oven to 350F [180C]. Grease two 8 by 2 in [20 by 5 cm] cake pans and line the bottoms with parchment paper.
• In a medium bowl or liquid measuring cup, combine the sour cream, buttermilk, oil, eggs, and vanilla.
• In the bowl of a stand mixer fitted with the paddle, mix together the flour, granulated and brown sugars, cocoa powder, baking soda, baking powder and salt on low speed until combined. With the mixer running on low, slowly add the buttermilk mixture. Increase the speed to medium and beat until combined, 20 to 30 seconds. Slowly pour the hot coffee into the batter and mix until just combined. Using a spatula, give the batter a couple of turns to make sure it is fully mixed.
• Divide the batter evenly between the prepared pans. Bake for 25 to 35 minutes, until a wooden skewer or toothpick comes out with the tiniest bit of crumb.
• Transfer the cakes to a wire rack and let cool for 30 minutes. Turn the cakes out onto the rack, remove the parchment paper, and let cool completely. Once cool, the cakes can be frosted right away or wrapped in plastic and refrigerated overnight.

For the chocolate buttercream

• Pour 1 in [5 cm] of water into a medium sauce-pan over medium heat and bring it to a gentle boil.
• Melt the chocolate in a heatproof bowl set over the pan of boiling water, being careful not to let the water touch the bottom of the bowl. Stir constantly until just melted and set aside to cool slightly.
• In the bowl of a stand mixer fitted with a paddle, beat the butter and salt on medium speed until light yellow and fluffy, about 3 minutes. Add the corn syrup and vanilla and beat on medium speed until combined. Lower the speed to low and gradually add the confectioners' sugar. Beat on low, stopping to scrape down the sides of the bowl as necessary, until smooth and creamy, 6 to 8 minutes. Add the cooled chocolate and mix on low speed until no streaks remain. Use a rubber spatula to mix the frosting a few more times, making sure it is completely combined.

To Assemble

• Arrange your first layer on a turntable or serving platter. Place (about) one heaping cup of buttercream on the first layer. Use an offset spatula to spread the buttercream evenly over the layer, all the way to the edges. Top the buttercream with the second layer of cake. Use the rest of the buttercream to frost the cake as desired. The cake can be covered and stored in the refrigerator for 24 hours. Bring to room temperature before serving.