Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 13x18-inch sheet pan (or a 10x15-inch jelly roll pan for a slightly thicker cake). Set aside.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, and salt. Make sure there are no lumps.
Step 3: In a medium saucepan, combine the butter, unsweetened cocoa powder, and brewed coffee (or water). Bring this mixture to a gentle boil over medium heat, stirring constantly, then immediately remove it from the heat. Pour the hot chocolate mixture over the dry ingredients in the large bowl and mix well with a whisk until smooth.
Step 4: In a separate small bowl, whisk together the buttermilk, eggs, baking soda, and vanilla extract. Pour this buttermilk mixture into the chocolate batter and mix until just combined. Be careful not to overmix, as this can lead to a tough cake.
Step 5: Pour the thin cake batter evenly into your prepared sheet pan. Bake for 20-25 minutes, or until a wooden skewer inserted into the center comes out clean. While the cake is baking, prepare the frosting.
Step 6: For the frosting: In a saucepan, melt the butter. Stir in the unsweetened cocoa powder and brewed coffee (or water). Bring to a boil, then remove from heat. Whisk in the powdered sugar, buttermilk, and vanilla extract until smooth. This should be a pourable, slightly thin frosting. If it's too thick, add a tiny bit more coffee; if too thin, a bit more powdered sugar.
Step 7: As soon as the cake comes out of the oven, while it's still hot, pour the warm frosting evenly over the entire surface of the cake. The warm cake will absorb some of the frosting, creating an incredibly moist layer. Allow the cake to cool completely before serving. The frosting will set as it cools, forming a delicious, slightly fudgy crust.
Step 8: Once cooled, slice and serve. For best results, store any leftovers at room temperature, well-covered, for up to 3-4 days. If you prefer, you can store it in airtight containers to maintain its freshness.