Preheat oven to 170 degrees C. Grease the bottom of a 20x30cm pan.
In a large bowl, combine flour and brown sugar. Cut in peanut butter and margarine until crumbly. Reserve 1 cup of crumb mixture. To the remaining crumb mixture, pour in milk, eggs, vanilla, baking powder and bicarb soda. Mix well and pour into pan. Sprinkle top with reserved crumb mixture and chocolate chips.
Bake in the preheated oven for 45 to 50 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.