Melt the margarine and chocolate together in a small bowl, stirring until completely melted. Place the melted chocolate and margarine mixture in a large bowl then the sugar and beat on medium speed for 2 minutes. Slowly mix in the beaten eggs, and beat until combined. Stir in the flour and vanilla beating just until fully mixed.
Pour the batter into the prepared pan and baked in the middle of the oven for 25 - 30 minutes, or until a tester inserted in the centre comes out clean.