Preheat the oven to 350°F. Spray a large sheet pan with nonstick spray and set aside.

In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt.

In a medium bowl, combine boiling water, butter, buttermilk, and vanilla. Stir until butter is melted and mixture is lukewarm.

Add the wet ingredients to the dry mixture and stir until combined.

In a separate bowl, beat eggs and then stir into the batter.

Pour the batter into the prepared sheet pan and bake for 10 minutes.

While the cake is baking, prepare the frosting by heating milk, butter, and vanilla, then stirring in cocoa powder and powdered sugar until smooth.

Once baked, spread the frosting evenly over the warm cake. Let it cool before serving.