Prepare the Crumb Topping: In a medium bowl, combine the plain flour, granulated sugar, light brown sugar, and cinnamon (if using). Add the cold, cubed butter. Using your fingertips or a pastry blender, work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining. This should take about 3-5 minutes. Set aside.

Preheat Oven and Prepare Muffin Tin: Preheat your oven to 200°C (180°C fan/Gas Mark 6). Line a 12-hole standard muffin tin with paper cases. Lightly spray the inside of the cases with a little non-stick spray, if desired, to prevent sticking.

Combine Dry Ingredients: In a large mixing bowl, whisk together the plain flour, granulated sugar, baking powder, bicarbonate of soda, and salt. Make sure there are no lumps and the mixture is well combined, creating a uniform, pale white colour.

Combine Wet Ingredients: In a separate medium bowl, whisk together the egg, buttermilk, vegetable oil, lemon zest, and vanilla extract until smooth and evenly combined. The mixture should appear slightly yellow from the egg and lemon zest, with a creamy consistency.

Combine Wet and Dry & Add Blueberries: Pour the wet ingredients into the dry ingredients. Using a rubber spatula, gently fold until just combined. Be careful not to overmix; a few lumps are absolutely fine. Overmixing develops gluten, leading to tough muffins. The batter should look thick and shaggy, with visible streaks of flour. Gently fold in the blueberries. The batter will be thick and sticky, speckled with vibrant blue berries.

Fill Muffin Tin and Add Topping: Divide the muffin batter evenly among the 12 prepared muffin cases, filling each nearly to the top. This generous filling is key to achieving those bakery-style domes. Generously sprinkle the prepared crumb topping over the batter in each muffin case. You should see a good layer of golden-brown crumbs on top of each muffin.

Bake: Place the muffin tin into the preheated oven. Bake for 5 minutes at 200°C (180°C fan/Gas Mark 6). This initial high heat helps create that dramatic rise. After 5 minutes, without opening the oven door, reduce the oven temperature to 175°C (155°C fan/Gas Mark 4) and continue to bake for another 20-25 minutes, or until the muffins are golden brown on top, a skewer inserted into the centre comes out clean, and they smell wonderfully sweet and lemony.

Cool: Remove the muffin tin from the oven and let the muffins cool in the tin for 5-10 minutes. The crumb topping will feel firm to the touch, and the muffins will have a beautiful golden hue. Then, carefully transfer them to a wire rack to cool completely. Allowing them to cool slightly in the tin helps them set before moving, preventing them from breaking apart.