Preheat your oven to 350°F (175°C) and grease a large sheet pan.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt.
In another bowl, whisk together buttermilk, vegetable oil, eggs, and black pepper until well combined.
Add the wet mixture to the dry ingredients and stir until just combined; do not overmix.
Carefully stir in the boiling water until the batter is smooth; it will be quite thin.
Pour the batter into the prepared sheet pan and bake for 30-35 minutes, or until a toothpick comes out clean.
While the cake is baking, prepare the frosting by melting the butter and combining it with powdered sugar, vanilla, and cream.
Once the cake is cool, spread the frosting evenly over the top.
Allow the cake to set for a while before slicing into squares and serving.