Preheat the oven to 180°C. Line a muffin tin with paper liners, set aside.
In a large bowl, whisk together the flour, sugar, baking powder, and bicarbonate of soda.
In a separate bowl, mix together the sourdough discard, buttermilk, melted brown butter, eggs, maple syrup, and vanilla extract. Using a spatula, gently fold the wet ingredients into the dry, stirring until everything is just combined. Fold through the frozen blueberries.
Spoon the batter into the prepared liners, filling about ¾ of the way up. Set aside.
Make the crumble by using your fingers to rub together the three ingredients in a small bowl. Sprinkle generously & evenly on top of the muffins.
Bake until golden brown and a skewer inserted in the center of the muffins comes out clean, about 15-18 minutes.
Remove the muffins from the oven & cool in the tin for 5 minutes before turning out onto a wire rack to clean completely.