The Best Brownies

These are the best brownies. They are so easy, and have just the right chewy consistancy...

150g Butter, Chopped
125 Dark chocolate, broken in to small pieces
3 eggs
375g sugar
1 tea spoon vanilla essence
125g plain flour
30g cocoa powder

1. Preheat the oven to 180C. Grease a 20cm square tin with oil, then line with greaseproof paper.
2. Put the butter and chocolate in a pyrez bowl and heat over a pan of water until melted and smooth. Don't let the bottom of the bowl touch the water. Remove from the heat and cool slightly.
3. Use a whisk to beat the eggs, sugar and vanilla together in a large bowl. Add the chocolate-butter mixture and whisk together. Stir in the combined sifted flour and cocoa. DO not over mix. Pour in to the tin and bake for 40mins until slightly risen and firm, but still squidgy in the middle.