Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9 inch round cake pan.
In a medium bowl, mix together the cocoa powder, flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the buttermilk; beat just until incorporated. Pour batter into prepared pan.
Bake in preheated oven for 30 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow cake to cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.
Interesting Facts
Baking time will vary at different altitudes. For high altitudes, baking time may need to be increased.
Cake can be stored in an airtight container for up to 3 days.