Preheat oven to 350°. In a bowl with a mixer, beat sugar, buttermilk, coffee, oil, eggs, salt, and vanilla on medium speed until well blended.
Whisk together flour, 1/2 cup cocoa powder, espresso powder, baking soda, and baking powder in a bowl to blend. Gradually add to wet mixture; beat on medium speed until smooth, scraping down sides as necessary. Stir in semisweet chocolate chunks.
Set baking molds on a rimmed baking pan and fill each about two-thirds full. Bake until a toothpick inserted in centers (avoid chocolate chunks) comes out clean, about 27 minutes. Cool cakes completely on a rack.
Melt bittersweet chocolate with cream in a small saucepan over very low heat and stir with a rubber spatula until smooth and blended. Remove from heat and let cool until thick but still spreadable, about 40 minutes. Spread on cakes. Top each cake with a few coffee beans and dust with cocoa. Let frosting firm up completely before serving, at least 30 minutes.
Note: Nutritional analysis is per cake.