Pre-heat the oven to 140 degrees. Grease a baking sheet.
In a mixing bowl, cream together the sugar and butter. Beat in the egg.
Incorporate the flour and blend well.
Add the cocoa powder and fold in. Do not over mix in as you want to create a slight marble effect.
Shape the mixture into balls, approx the size of a golf ball.
Place on the baking sheet and with the palm of your hand, flatten out, not too thin.
Bake in the oven for approx 20-22 mins. The cookies will be cooked when firm on the top. Don't worry if slightly soft to the centre as they will harden when cool. Place on a wire rack to cool down.
Depending on the size of your baking sheet, you may need to bake in two batches.