Bakery Style Blueberry Streusel Muffins

PREPARE: Preheat the oven to 220°C / 425°F / Gas 7. Grease and line a 6-cup large-capacity muffin pan or a 12-cup standard muffin pan with baking paper squares or muffin cups.
MAKE THE STREUSEL: Melt the butter in a small saucepan, then in a small mixing bowl, whisk together the flour, sugar, hazelnuts, cinnamon and salt. Drizzle in the butter and use a fork to stir just until the mixture is rough and pebbly. Transfer to the freezer.
MAKE THE BATTER: In a large bowl, stir together the flour, sugar, lemon zest baking powder, and salt. In a separate small bowl, toss the blueberries with two tablespoons of the mixture. In a jug, whisk together the buttermilk, vegetable oil, eggs, and vanilla until well combined.
COMBINE: Pour the buttermilk mixture into the dry ingredients. Stir until just combined. Fold through the blueberries, then spoon into the prepared muffin pan, taking care not to overfill the cups (see notes). Sprinkle streusel topping generously over the top of the muffins, breaking up any overlarge chunks with your fingers, the streusel should be pebbly, coarse crumbs.
BAKE: Bake in the lower third of the preheated oven for 5 minutes, then reduce the heat to 190°C / 375°F / Gas 5. For large muffins, bake for 30 minutes until risen and golden brown. For standard muffins, check around 15 minutes to ensure they are risen and cooked through.
COOL: Allow to cool in the pan for 5-10 minutes, then transfer to a wire rack to cool completely.