Preheat the oven to 350°F. Grease and flour a 9x13 baking pan.
In a large bowl, cream together the butter, cream cheese, and sugar until light and fluffy. Beat in the eggs one at a time.
In a separate bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Add to the butter mixture alternately with the buttermilk, beginning and ending with the flour mixture.
Stir in the vanilla and chopped chocolate. Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Cool completely before serving.
Interesting Facts
Philadelphia Red Cake is also known as Red Velvet Cake.
This cake is traditionally served with a cream cheese frosting.
The cocoa powder gives this cake its signature rich, chocolatey flavor.