Preheat oven to 180 C / Gas 4. Grease a 12-hole muffin tin or line with paper cases.
For streusel: Whisk 30g wholemeal flour, 30g plain flour, 2 tablespoons caster sugar, 2 tablespoons dark brown soft sugar, cinnamon, nutmeg and salt together in a small bowl. Stir in 3 tablespoons melted butter until crumbly; set aside.
For muffins: Stir egg, 50g dark brown soft sugar, 3 tablespoons caster sugar, 70g melted butter and soured cream together in a bowl. Whisk 125g wholemeal flour, 60g plain flour, baking powder, bicarb and 1/4 teaspoon salt together in a separate large bowl.
Stir soured cream mixture, rhubarb, and about 1/3 of streusel mixture into flour mixture just until ingredients are moistened. Divide into the prepared muffin tin. Sprinkle the remaining streusel over the tops of muffins.
Bake muffins in the preheated oven until a skewer inserted near the centre comes out clean, about 20 minutes. Cool in the tins for about 5 minutes before removing.