Preheat the oven to 200 ° C and grease the muffin tin.
For the crumble, process all the ingredients in a food processor to a crumbly mass.
For the dough, mix the flour, sugar, baking powder and salt in a bowl. Whisk the egg and stir in the buttermilk and melted butter, then add this to the flour mixture and work into a batter and fold in the blueberries.
Fill the muffin molds ig full with the batter and spread 2 teaspoons of streusel on top.
Bake the muffins on the middle rack for about 30 minutes. Let cool for 15 minutes, then turn over and let cool completely.