Preheat oven to 180 degrees C. Grease a 12 hole muffin tin or line with paper cases.
Streusel Topping: Whisk wholemeal flour, plain flour, caster sugar, dark brown soft sugar, cinnamon, nutmeg and salt together in a small bowl. Stir in the melted butter until crumbly, set aside.
Muffins: Stir egg, dark brown soft sugar, caster sugar, melted butter and sour cream together in a bowl. Whisk wholemeal flour, plain flour, baking powder, bicarb and salt together in a separate large bowl.
Stir the sour cream mixture, rhubarb and about 1/3 of the 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.