Preheat the oven to 350 degrees F (180 C), line a muffin tin with muffin liners, and set it aside.
Add the butter and sugar to the Thermomix mixing bowl, and mix 1 minute/speed 3.
Then, add the egg and beat again 1 minute/speed 3.
Add the salt, vanilla extract, and milk, and mix 1 minute/speed 3,5.
Add the flour, and baking powder, and mix 1 minute/speed 2.
Next, add the blueberries to the batter and gently mix with the help of a spatula.
Pour the batter into the muffin tins, filling them for about 3/4 of the way full, add more blueberries on top, and sprinkle with demerara sugar. Bake for about 20 minutes or until a toothpick comes out clean.