Decadent and intensely fudgy, this brownie uses a riff on the simple one-bowl Baker's chocolate brownie recipe for the base. I topped it with a tangy, sweet chocolate buttermilk frosting. They are better when chilled slightly—but it's hard to wait that long to take a bite! —Posie (Harwood) Brien
Preheat the oven to 350° F. Line a 9- by 13-inch baking pan with foil and grease with butter or baking spray.
In a large glass bowl, microwave the chocolate and the butter on high, checking and stirring after each 30-second interval, until the butter melts (about 2 minutes). Stir until the chocolate is fully melted.
Add the sugar and mix well. Add in the eggs and vanilla and stir until combined. Add the flour and mix until well-incorporated.
Pour the batter into the prepared pan and bake for about 30 minutes. The top should be crackly and a tester inserted into the middle should come out clean. Do not overbake! Remove from the oven and let cool in the pan.
To make the frosting, add the butter, buttermilk, and cocoa powder to a small saucepan. Bring to a boil, whisking until smooth, and then remove from the heat. Transfer to a stand mixer (or large bowl) and add in the confectioners' sugar and the vanilla. Beat the mixture until fluffy and thick. **You might need less or more sugar, depending. Start with 2 cups, and add more if the frosting looks too thin. Also, if it isn't getting fluffy, try refrigerating the frosting for 20 minutes, and then beating it again.**
Spread the frosting over the brownies. Let the frosting set (it is helpful to chill them for a little while in the refrigerator), and then slice and serve.