Preheat the oven to 200°C (fan) / 220°C (conventional). Line a large baking sheet with greaseproof paper.
Measure the butter onto a plate and microwave for 10 seconds, just to soften it a little. Cut it into cubes and place in a large mixing bowl with both sugars.
180 grams Cold unsalted butter, 180 grams Light brown sugar, 80 grams White caster sugar
Beat using an electric mixer or wooden spoon until just combined (small flecks of butter are fine). Try not to over-mix at this stage otherwise the butter will become too soft.
Add one whole egg (yolk and white), and one egg yolk (discard the white). Add the vanilla extract. Mix to combine.
2 medium Eggs, 1 tsp Vanilla extract
In a separate bowl, whisk the self-raising flour, plain flour, salt and cornflour to combine.
200 grams Self-raising flour, 200 grams Plain flour, ¾ teaspoon Salt, 1 teaspoon Cornflour
Add the dry ingredients to the butter mixture and fold to make a thick cookie dough.
Fold in the white chocolate chunks.
150 grams White chocolate