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1
Adjust oven rack to middle position and preheat oven to 350u00b0F. Lightly spray 13- by 9-inch cake pan with baking spray. Line pan with parchment paper lengthwise with one sheet, then crosswise with a second sheet to create a sling; spray once again with baking spray.
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2
In large bowl, whisk together brown sugar, flour, cocoa powder, baking powder, baking soda, and salt. In medium bowl, whisk together eggs, sour cream, oil, and 2 teaspoons vanilla. Add wet ingredients to dry ingredients and stir until mixture is a thick paste.
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3
Stir espresso powder into boiling water, then, whisk into batter until smooth.
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4
Place chocolate in small bowl and microwave until smooth and melted, 1 to 2 minutes, stopping microwave to stir chocolate every 30 seconds. Stir melted chocolate into batter.
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5
Scrape batter into prepared pan. Bake until a toothpick inserted in the center of cake comes out clean, 20 to 25 minutes. Transfer cake to cooling rack and cool in pan 10 minutes. Pull cake out onto a cooling rack using the parchment sling and cool completely, about 1 hour.
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6
Meanwhile, pulse Cocoa Puffs and milk powder in food processor until powdery. Set aside.
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7
Place whisk attachment on electric mixer. Whip cream cheese and butter on medium speed until light and fluffy, about 2 minutes. Add remaining 1 teaspoon vanilla and whip just until incorporated.
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8
Reduce speed to lowest setting and add confectioners' sugar. Whip until incorporated, scraping bottom and sides of bowl with rubber spatula as needed, 30 to 60 seconds. Increase speed to medium-high and continue to whip until frosting is light and fluffy, 2 to 3 minutes.
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9
Reduce speed to lowest setting once again; add Cocoa Puffs-milk powder mixture and whip until combined, about 1 minute.
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10
Frost top and sides of cooled cake. Serve.
