Preheat your oven to 170°C/350°F.
Measure out your dry ingredients (except the sugar) and add to a mixing bowl. Lightly whisk together until evenly dispersed.
Melt the butter in the microwave. Add the sugar and mix vigorously until the mixture is reasonably smooth. Add in the vanilla extract and stir through. Leave to cool slightly.
Lightly beat your egg and extra yolk before adding to the sugar and butter mixture.
Pour your wet mixture into the dry, stirring with a wooden spoon to combine. When the dough becomes too stiff to stir, work it with your hands. It should be a nice, pliable texture and not too sticky.
Take meatball-sized chunks of dough and roll in your hands, before flattening into cookie shapes. this mixture doesn't spread, so if you leave it in a ball it will make something more akin to chocolate chip rock cakes. You should get around 20 decent-sized cookies.
Bake in the oven for around 20 minutes, until the very edges are crisp. They will seem too soft, but will firm up considerably as they cool. Don't overbake or they will become hard and biscuity as opposed to chewy.
Leave to cool on the baking sheet until cool enough to touch, then transfer to a wire rack. Store in an airtight container for up to 4 days, or freeze.