Preheat the oven to 200°C/400°F (180°C fan) and line a 12 hole muffin tin with muffin cases.
If the butter is chilled, melt it in the Thermomix bowl for 40 sec/50℃/speed 4 to soften. (Omit this step if the butter is already at room temperature). Then add the flour, brown sugar, baking powder, milk, eggs and vanilla extract to the Thermomix bowl and mix for 6 sec/speed 5. Scrape down the bowl and mix again for 6 sec/speed 5. Stir in 100g of the blueberries by hand so not to squash them. Reserve the rest for sticking into the top of each muffin before baking.
Spoon or scoop the batter into the muffin cases about two-thirds of the way up. Top the muffins with the reserved blueberries and bake in the oven for 15-20 minutes until lightly golden and a toothpick comes out clean. Leave in the tray for 5 minutes before turning out onto a wire rack to cool.
Enjoy them whilst they're still warm from the oven.
Notes
Blueberries: Frozen blueberries also work in this recipe. Don't thaw them first as their juices bleed into the batter and you'll end up with a purple muffin!