1. Preheat the oven to 375°F. Grease and line the base of a 11 x 7 tin with baking parchment paper. Lightly grease the sides.
2. Place 1/2 cup of the chocolate chips with the butter in a heatproof bowl. Melt the white chocolate chips and butter, stirring gentle. Remove from the heat and leave to cool slightly.
3. Place the eggs and sugar in a large bowl and beat until light and foamy. Whisk in the melted chocolate mixture.
4. Sift over the flour and fold in with the metal spoon with the walnuts, vanilla extract and the remaining chocolate chips.
5. Spread out the mixture in the tin with a palette knife and bake for 30 minutes, or until risen and brown.
6. Remove from oven and let cool. Cut into 12 bars.
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