Place the butter in a small pot over a low heat. Cook, stirring intermittently, until the butter is a medium brown colour and smells deeply nutty.
Measure the sugar into a small mixing bowl on the scales. Tare the scale and pour in the brown butter, which should weigh 60-65g once browned. Make sure to scrape all of the little brown butter solids into the bowl as they contain all the flavour.
Add the milk, vanilla bean paste and salt. Adding the milk before the tapioca flour is CRITICAL to ensure it doesn't develop a pasty and chalky texture. I made this mistake a few times while developing the recipe - never again. Milk MUST go in first or you'll ruin the batch.
Mix to combine, then gradually add the heat treated flour. Stir it in and continue to add flour until the dough is no longer oily. It should have a firm (but not too firm) cookie dough texture and you should be able to pick up a ball of it in your hands. If it is firmer than this, crumbly or floury, add a dash of milk or water to correct the consistency. Add milk to correct the consistency if the dough is too firm for you, rather than adding less flour.
Once you are happy with the consistency, stir in the finely chopped chocolate. You can eat as is or chill in an airtight container in the fridge for 20 minutes or so.
Leftover cookie dough can be stored in the fridge in an airtight container. It will firm up in the fridge, so you will need to warm it up (I roll the ball in my hands) to restore it to it's former texture.
Best eaten within a few days.