Preheat oven to 180 degrees Celsius (fan-forced).
Line a 12-hole muffin tray with paper cases and set aside until needed.
Place the plain flour, bicarbonate of soda and cocoa powder into the Thermomix bowl and mix for 5 seconds, Speed 8.
Add the caster sugar, salt, eggs, sour cream, vegetable oil, milk and vanilla extract and mix for 5 seconds, Speed 5. Scrape down the sides of the bowl and repeat for a further 5 seconds, Speed 5. The batter should be slightly lumpy.
Use the spatula to gently fold the milk chocolate chips through the muffin batter; do not over-mix.TIP: Reserve ¼ cup of the chocolate chips for sprinkling on top of the muffins before baking (optional). Spoon the mixture into the paper cases, filling ⅔ to the top.Sprinkle with the reserved chocolate chips (optional). Bake for 20-25 minutes, using a skewer to check at 20 minutes - see notes.
Allow to cool in the baking tray for 5 minutes before transferring to a wire rack to cool completely.