Preheat oven to 180 degrees celsius (fan-forced). Grease and line baking trays and set aside (use as many as will fit in your oven).
Place room temperature butter and caster sugar into the Thermomix bowl. Cream 1 minute, Speed 5 (scrape down the sides after 30 seconds).
Add the condensed milk and mix for a further 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 each 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.