Preheat oven to 180 degrees & grease a square baking tray. Place butter, sugar, chocolate and golden syrup in a saucepan and heat gently until smooth. Remove from heat and leave to cool. Beat eggs and vanilla then whisk in the chocolate mixture. Fold in sifted flour, cocoa and baking powder using a metal spoon. Pour mixture into baking tray and bake for 25 minutes until top of cake is a bit crisp and it's starting to shrink away from edges of the tray. Inside should still be soft and gooey, be careful not to overbake. Leave to cool then cut into squares.