Instructions

1. Preheat oven to 350°F and spray a 13 x 9-inch baking pan with nonstick spray.
2. Whisk together flour, cocoa, baking powder, baking soda, and salt in a large bowl; make a well in the center of the mixture. Add sugars, eggs, egg yolk, milk, yogurt, oil, and vanilla to flour mixture; whisk in just until incorporated. Dissolve instant espresso in boiling water; add to batter and whisk in just until smooth. Fold in chopped chocolate.
3. Pour batter into prepared baking pan and smooth with an offset spatula; tap pan sharply to reduce air bubbles.
4. Bake for 24 to 26 minutes, until cake springs back to the touch, a toothpick inserted in the center comes out with a few crumbs, and the edges are pulling slightly away from the pan. Let cool completely in pan on a wire rack.
5. For the frosting, beat butter and salt in a large mixer bowl on medium speed until combined. With the mixer on low speed, gradually add sifted cocoa, beating until incorporated. Mix in vanilla. Add powered sugar 1 cup at a time, alternating with 1 tablespoon of heavy cream. Scrape down the bowl as needed. Add remaining 2 tablespoons of heavy cream and mix on low speed until combined, then beat on medium speed for 1 minute, until light and airy.
6. Dollop frosting over cooled cake and swirl with an offset spatula. Chill cake for 20 minutes in the refrigerator before slicing.