Put the margarine into a pan and melt it over a low heat. Pour it into a mixing bowl, then add the sugar and vanilla.
Break the eggs into a small bowl and beat them. Add them to the large bowl, a little at a time. Beat them in well.
Sift the flour into the bowl and add the baking powder and cocoa. Stir everything together so that it is mixed well.
Put the chocolate on to a chopping board and cut the chocolate into small pieces. Add them to the mixture and stir again.
Pour the mixture into the tin and smooth the top with the back of a spoon. Bake for 45 mins.
22 x 30 x 2.5cm (9 x 12x1 in) oblong tin lined with greaseproof paper and cook at 180C gas mark 4 for approx 45 minutes
The brownies are ready when they have risen and a crust has formed on top. They should still be soft in the middle.
Leave the brownies in the tin for 5 minutes then cut into approximately 16 squares. Leave them to cool on a wire rack.
Keep in an airtight container and eat within the week.
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