For the cake

1. Pre-heat oven to 325 degrees. Prepare a 13 by 9-inch baking dish, by either lining with parchment and greasing with nonstick spray, or just spray the dish with nonstick spray. Lightly dust the baking dish with flour and knock off any excess, leaving a very thin layer.
2. Sift flour, Dutch-processed cocoa, espresso powder, and salt into a large bowl and set aside. Microwave the semisweet chocolate at 50% power for 2 minutes. Stir, add butter, and continue to heat it at 50% until melted, stirring once every additional minute. Whisk the eggs, sugar, and vanilla together in a medium bowl.
3. Whisk the chocolate and butter mixture into the egg mixture until thoroughly combined. Combine buttermilk and baking soda, and whisk into the chocolate mixture, then pour into dry mixture and whisk until batter is smooth and glossy. Pour the batter into your prepared baking dish/pan and smooth the top with a rubber spatula. Bake until firm in center when lightly pressed and toothpick inserted in center of cake cokes out clean, about 30-40 minutes (mine took 35), rotating pan halfway through baking. Let cool on wire rack until room temperature, about 2 hours.

For the frosting

1. Heat heavy cream in a large measuring cup in the microwave for about 3-4 minutes, until you see a very gentle simmer around the edges of the cup.
2. Pour the hot cream over the milk chocolate and espresso powder, and leave it alone for 2 minutes. After 2 minutes, gently whisk until the mixture combines. Let cool at room temperature for about 2 hours, or place in your fridge, and let it cool (don’t let it harden), about a half hour. When the mixture is cool but still very smooth, whip it up using a hand-held electric mixer until lightly and fluffy, about 2-3 minutes. Dollop the frosting on top of the cooled cake, and spread evenly. Cut into squares, and sprinkle tops with a very tiny pinch of flaky sea salt.
3. Enjoy immediately. Store leftovers in an airtight container.