1. Preheat oven to 200°C (fan-forced 180°C) and line a 12 count muffin tin with papers.
2. To make the crumble, place the flour, CSR Dark Brown Sugar and butter in a small bowl. Rub in the butter using your fingertips until the mix is a fine crumb. Blend muesli in a food processor to a crumb consistency and then add to the flour mix. Place in fridge until ready for use.
3. To make the muffins, place the butter, buttermilk, CSR Caster Sugar and egg in a large bowl and whisk to combine well. In a separate bowl, place the flour, bicarbonate soda, baking powder and salt and whisk to combine. Add the blueberries and stir to cover with flour. Gently fold the dry ingredients into the wet ingredients until just combined. Batter will appear lumpy which is correct.
4. Spoon batter evenly into papers and sprinkle each with a bit of the chilled crumble mix. Bake for 20 minutes or until muffins are golden and firm on the top.