Lightly grease a 20cm/8inch shallow brownie tin and line with grease proof paper.
Place the butter, sugars, dark chocolate and the golden syrup in a heavy based saucepan and melt on a low heat until all ingredients are well blended. Take off heat and leave to cool.
Beat together the eggs and vanilla extract. Then add to the cooled chocolate mixture.
Sieve together the cocoa powder, flour and baking powder together into the egg and chocolate mixture and fold in carefully making sure all the flour has been incorporated into the wet mixture.
Spoon the mixture into the prepared tin and bake into a pre-heated oven of 180 degrees / 160 degrees fan / Gas Mark 4 for 25 minutes until the top is crispy and the edge of the cake is starting to shrink away from the sides. The inside of the cake will still be quite soft to the touch and a little stodgy.
Leave the cake to cool before removing from the pan. Cut into 9 squares.