Pre-heat your oven to 180 degrees celsius (fan-forced) and line 2 trays with baking paper.
Place the softened butter, caster sugar and sweetened condensed milk into your Thermomix bowl and mix for 1 minute, speed 5. Scrape down the sides of the bowl and mix for a further 40 seconds, speed 5 or until pale and creamy.
Add the self raising flour and mix for 30 seconds, speed 6. You will need to use your spatula to push the flour down towards the blades.
Scrape the sides of the Thermomix bowl and mix for a further 30 seconds, speed 5 or until combined.
Add the chocolate chips (if using) and mix for 30 seconds, speed 4, on reverse). You will again need to use the spatula to push the ingredients down towards the blades.
Use a teaspoon to scoop up the mixture and roll into balls. Place onto the prepared baking trays and lightly press with the back of a fork to slightly flatten.
Place the baking trays into the oven to cook for 12 - 15 minutes or until the biscuits begin to turn golden on the edges.
Carefully remove from the oven and let the biscuits cool for 5 minutes on the baking trays before transferring to a wire rack to cool completely. Repeat from step 6 until all of the mixture has been used.