Preheat oven to 180°C/350°F (160°C fan forced). Grease one 13-inch by 9-inch rectangular cake pan with butter or oil. Line with parchment paper, then butter and dust the pan using cocoa powder. See note 3 for pan substitutions.
Place the sugar in a large bowl. Sift the flour, cocoa, baking soda, baking powder, and salt into the same bowl. Use a fork or a whisk to mix all the dry ingredients together thoroughly.
Make a well in the center of the flour mixture. Add all the wet ingredients. Use a spatula or a spoon to just barely mix the dry and the wet ingredient. This is so that the flour doesn't start flying around when you start the electric whisk.
Beat for two minutes, using a spatula to scrape the bowl as needed. Please note that the batter will not be thick as regular cake batters, but will be a bit liquid.
Pour the batter into the prepared pan and bake for 35 to 40 minutes, until a cake tester comes out clean. Cool in the pan for 30 minutes, then turn it out onto a cooling rack and cool completely.
Dust with powdered icing sugar or cocoa powder, and serve. Can also be served with a dollop of cream or ice-cream. You can also frost with your favourite buttercream frosting or chocolate ganache.