Preheat oven to 180 degrees celsius (fan-forced). Grease and line 3 baking trays and set aside (or as many trays as will fit in your oven).
Place the butter and caster sugar into the Thermomix bowl. Mix for 1 minute, Speed 5 (scrape down the sides after 30 seconds).
Add the condensed milk and mix for 30 seconds, Speed 5.
Add 250g of the self-raising flour and mix for 20 seconds, Speed 6. Scrape down the sides of the bowl. Add another 250g of the self-raising flour and mix for a further 20 seconds. Add the final 250g and mix for 20 seconds, Speed 6 (please note that you'll need to use the spatula to assist with the mixing).
Divide the cookie dough between bowls and flavour however you'd like (see notes).
Roll into teaspoon-sized balls and place onto the prepared baking trays. Press down lightly with a fork.
Bake for 10 minutes. Allow to cool for 10 minutes on the baking trays before transferring to a wire rack to cool completely. Store in an airtight container at room temperature for up to a week.