In a medium bowl, cream your margarine and sugar until the are light and fluggy, and then beat your eggs into this mixture.
Next, stir in the flour a little at a time, and then add the baking powder.
Stir in milk and coffee extract, combining until it forms a smooth batter.
Spoon this mixture into the prepared heatproof bowl, and cover the bowl with a pleated piece of baking parchment, and then a pleated piece of foil, making sure to form it around the bowl, and then secure with string.
Place bowl in a steamer or large pan and half fill steamer or pan with boiling water.
Cover the steamer or pan for 1-1 1/4 hours, until cooked through.
For the sauce, put milk, brown sugar and cocoa in a medium saucepan and heat on low just until the sugar dissolves.
Blend the cornstarch with 4 tablespoons cold water to make a smooth paste, and stir into the pan.
Turn heat up (med-high) and bring to a boil, stirring until smooth and thickened.
Reduce to low-med and cook over gentle heat for 1 minute.
When 1-1 1/4 hour (s) has passed, remove the string, foil, and paper, and turn the Sponge pudding out onto a serving plate and spoon sauce over the top.
(NOTE: the pudding is covered with pleated paper and foil to allow it to rise. It will not rise if you do not do this properly.) The foil reacts with the steam and must therefore not be placed directly against the pudding- be sure to place the paper under the foil.