Step 1

Preheat oven to 350℉. Line an 8x8 square baking dish with parchment paper. Spray the pan liberally with nonstick spray. Set aside.

Step 2

In a large mixing bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Add the eggs, sugar, vanilla, vegetable oil, and buttermilk, and whisk until smooth. Slowly pour in the hot coffee, whisking to combine, until batter is smooth and pourable. Pour batter into prepared baking dish. Bake for 30-35 minutes, or until center is set and no longer jiggly, and a toothpick inserted in the center comes out clean. Let cool for 30 minutes in the pan, then carefully remove and let cool on a cooling rack for the remainder.

Step 3

To make the frosting, add the softened butter to the bowl of a stand mixer fitted with the paddle attachment. Beat for 2-3 minutes on medium-high speed, until light and fluffy. Lower the speed to medium, and gradually add the powdered sugar. Once incorporated, increase speed to medium-high, and beat for an additional 1-2 minutes. Add the vanilla and heavy cream, and beat for an additional 30 seconds. Turn off the mixer and add the melted chocolate. Increase speed to medium-high and beat for an additional 1-2 minutes or until incorporated, scraping down the sides as needed. Add the flaky salt and beat for 30 seconds or until combined.

Step 4

Frost the cooled cake with the frosting, using an offset spatula to create the frosting "swoop" pattern, if you wish. Decorate with sprinkles, and sprinkle with flaky sea salt to finish. Cut into squares and enjoy!