Melt the chocolate with 25 g (1 oz) of the butter in a bowl over a pan of simmering water (see
Cut the rest of the butter into small cubes, mix with the the flour and baking powder in a food processor and blend to crumbs. Add the sugar, lemon rind, eggs, white chocolate and bananas. Whiz until well combined.
Grease a 1.8 litre (3 pint) ring tin. Line the base and sides with nonstick baking paper. Spoon one-quarter of the cake mixture into the base, then drizzle over one-third of the chocolate. Continue layering the cake mixture and chocolate, finishing with cake mixture.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 50–60 minutes until the cake feels firm when lightly pressed. Leave in the tin for 10 minutes, then loosen the edges with a knife and turn out on to a wire rack to cool.
Melt the white chocolate in a pan over a bowl of simmering water. Drizzle over the top of the cake. Decorate with the white chocolate chips.