Preheat your oven to 425F (220C).
Make the crumble: combine the melted butter, sugar, and flour in a bowl just until combined. Place in the fridge.
Moving to the muffins: in a bowl add melted butter, olive oil, eggs (Note: Eggs are mentioned in instructions but not in ingredients list) and sugar. Whisk until smooth and combined.
Add milk, sour cream, vanilla, and orange zest. Mix to combine.
Add flour, baking soda, baking powder, salt, and cinnamon and fold them in, until almost fully incorporated.
Add the blueberries and fold gently just until combined, don’t over-mix.
Scoop the batter into muffin tins lined with muffin liners.
Crumble the mixture from the fridge generously on top of the muffins.
Bake for 5 minutes at 425F (220C). Then reduce the temperature to 350F (170C) and bake for an additional 15-18 minutes, or until it passes the toothpick test.
