Preheat the oven to 350°F (175°C). Grease and flour a 9×13 inch cake pan.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
Add the softened butter, buttermilk, eggs, and vanilla extract to the bowl. Use an electric mixer to beat the mixture on medium speed for 2-3 minutes, until it’s smooth and creamy.
Pour the batter into the prepared cake pan and bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
While the cake is baking, make the frosting. In a medium saucepan, melt the butter over low heat. Add the cocoa powder and stir until it’s smooth. Add the milk and vanilla extract and stir again. Remove the saucepan from the heat.
Gradually add the powdered sugar to the saucepan, stirring until the frosting is smooth and well combined.
When the cake is done baking, remove it from the oven and let it cool for 10 minutes.
Spread the frosting evenly over the top of the cake.
Serve and enjoy your delicious Neiman Marcus Cake!