Preheat oven to 375 degrees. Lightly coat an 20 cm round cake pan with non-stick cooking spray. With an electric mixer cream the margarine and sugar in a mixing bowl. Beat in the egg. Stir in the buttermilk. Set the bowl aside.
In a large bowl, combine the cocoa, salt, baking soda, baking powder, and flour, and mix together well. Add the creamed margarine-buttermilk mixture to the dry ingredients in one batch. Stir until the dry ingredients are thoroughly moistened. Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 35 minutes, or until a cake tester comes out dry. Remove from the pan and let cool on a rack.