- 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.