Preheat oven to 175°C (347 °F)
Whisk together the butter, sugar and vanilla, then add the eggs one at a time.
Sift together flour and baking powder.
Add to the previous mixture in batches, alternating with the milk while stirring. Start with flour and finish with flour.
Just before baking, add the blueberries to the batter.
Put in a piping bag and fill the muffin-cups.
Spoon the crumble over the top and bake for 25 min. Check for doneness with a skewer; it should come out dry.
If you're making a large quantity, divide the batter into several portions and only add the fruits just before baking. If you leave the fruits in the batter for too long, they will color it.