Preheat the oven to Gas 6, 200°C, fan 180°C. Grease and line a 18x28cm (7x11in) tin with nonstick baking paper.
Melt the chocolate and butter in a small pan over very low heat until just beginning to soften. Stir off the heat until melted.
Whisk the eggs and sugar together in a bowl and stir in the chocolate mixture. Sift in the flour and baking powder. Add the vanilla extract and maple syrup and stir to combine.
Spoon mixture into the prepared tin and spread to the corners. Bake for 25 minutes. Remove from the oven and allow to cool before cutting into pieces.
Melting white chocolate has to be done very carefully and slowly and will continue to soften once taken off the heat.
Please note: this recipe has been created and uploaded by a member of the Tesco Real Food online community. Tesco does not test customer recipes so cannot accept responsibility for the content, validate the results or answer queries regarding them.