Preheat the oven to 180°C, gas mark 4. Line a 21cm square cake tin with baking parchment. Put the pecans on a baking tray and bake for 10 minutes, until golden and fragrant. Set aside. Meanwhile, melt the butter and both sugars in a pan over a low heat, stirring to combine. Take off the heat and allow to cool slightly.
Beat the eggs, then whisk into the butter and sugar mixture along with the vanilla. Beat in both flours and the salt. Very roughly chop the white chocolate and the pecans, then stir though the mixture. Put into the tin and bake for 20/25 minutes, until risen and golden on top. Allow to cool completely in the tin before slicing into 16 squares.
