Make the cake:
1. Slide a rack in the middle of the oven and crank the heat to 350 degrees F (175 degrees C). [Editor's Note: Make that 325 degrees F (160 degrees C) if you're using dark non-stick pans.] Butter two 8-inch cake pans and line the bottoms with parchment paper. Butter and flour the paper, tapping out any excess flour.
2. Melt the chocolate, either in a microwave on medium or in a pot set over another pot filled with an inch of simmering water. (Make sure no water splashes into the chocolate or it'll seize and become grainy.) Set aside to cool until just warm.
3. Meanwhile, dump the sugar, flour, baking soda, and salt in the bowl of a stand mixer fitted with a paddle attachment.
4. Whisk together the hot expresso, sour cream, canola oil, butter, and vanilla and chocolate extracts in a medium bowl. Pour this coffee-sour cream brew into the dry ingredients in the stand mixer and mix on low until just blended, about 30 seconds. Slowly drizzle in the beaten eggs and mix on medium until smooth. Plop the melted chocolate into the bowl and beat until the batter is uniform in color, about 15 seconds. The batter will be thin, but that's how it should be.
5. Divide the batter evenly between the 2 pans and bake for 30 to 40 minutes, until a tester comes out clean. Transfer the cakes to a wire rack to cool for 10 minutes. Run a sharp knife around the edge of each cake and invert them onto another rack to cool completely.
Make the mocha frosting:
6. Dissolve the coffee granules in the water. Pour the coffee mixture into a food processor and add the butter, sugar, and heavy cream. Process the mixture until light and fluffy, about 5 minutes. The frosting will look hopelessly curdled and destroyed at first, but then just when you're about to lose all faith, it will come together beautifully. Honest.
Frost the cake:
7. Frost the cake, as usual. Devour immediately, as usual.
Recipe used with permission, David Leite. Sour Cream Mocha Cake Recipe © 1999 Joseph Moran. Photo © 1999 Emily Sandor. All rights reserved.