Preheat the oven to 180°c / gas 4. Grease 8in square baking tray and set aside.
In a bowl, beat the egg, margarine, sugar and lime zest in a bowl until thick and pale.
Sift both flours into the bowl and stir until well combined. Add half of the white chocolate buttons and mix well. Spoon the mixture into the prepared tin and bake for 20mins or until golden.
Allow to cool in the tin for 15mins.
Lift out the tin and while still warm, cut into fingers. Arrange on a wire rack and allow to cool completely.
Meanwhile, put the remaining chocolate buttons in a saucepan over low heat and cook for 16-18mins or until the chocolate is melted and smooth. Whilst still hot, drizzle the chocolate over each finger in a rough zigzag pattern and allow to set before serving.
