Firstly, line a 6-hole muffin tin with muffin cases. Then preheat the oven to 215C/205C Fan/Gas Mark 7.
For the muffins: Place the butter and both sugars into a large bowl and beat together until creamy. Then add the yoghurt, vanilla, egg and mix until combined. Sift in the dry ingredients and mix until just combined. Add the milk and mix again until just combined. Finally, stir in the blueberries, then divide the batter between the 6 muffin cases. Set aside while you make the crumble.
For the crumble: Place the chilled diced butter into a medium bowl with the sugar and flour. Rub the mixture between your fingertips until you’re left with a breadcrumb style texture – leave some crumbs larger than others. Sprinkle the crumble over each muffin batter.
Bake the muffins for 5 minutes. Then turn down the temperature to 175C/155C Fan/Gas Mark 4 and continue baking for another 18-20 minutes. The muffins will be golden and a toothpick inserted into the centre should come out clean. Let the muffins cool in their tins for 10minutes, then remove and place on a cooling rack to continue cooling.