For the cinnamon crumble:
1/2 cup all-purpose flour
1/4 cup packed brown sugar
Scant 1/2 tsp. ground cinnamon
2 ounces (1/4 cup) cold unsalted butter, cut into small pieces

For the muffins:
Butter and flour for the muffin tins, or paper muffin liners
6-3/4 oz. (1-1/2 cups) all-purpose flour
2 oz. (1/2 cup) cake flour
1 tsp. baking soda
1/2 tsp. baking powder
1/2 tsp. salt
3/4 tsp. ground cinnamon
1/2 tsp. grated or ground nutmeg
4 oz. (1/2 cup) unsalted butter, at room temperature
1 cup granulated sugar
2 large eggs
1-1/2 tsp. pure vanilla extract
1 cup buttermilk
1 cup fresh blueberries, picked over, or frozen blueberries