Preheat oven to 350F (175C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the buttermilk, oil, eggs, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes.
Gradually add the boiling water to the batter, mixing until just combined. The batter will be thin.
Pour the batter into the prepared pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out with a few moist crumbs attached.
Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.
Optional Fudge Frosting: While the cake cools, prepare the frosting. In a saucepan, melt the chopped chocolate and butter over low heat, stirring until smooth.
Remove from heat and whisk in the powdered sugar, milk, and vanilla extract until smooth.
Pour the frosting over the cooled cake and spread evenly. Let the frosting set before slicing and serving.
Tips: For a richer flavor, substitute strong brewed coffee for the boiling water. For a chocolate ganache frosting, use equal parts heavy cream and chocolate. Cake can be stored in an airtight container at room temperature for up to 3 days.