• Preheat oven to 220°C (425°F). Line a jumbo 6-cavity muffin pan with muffin liners (store-bought or homemade tulip liners will work), or generously grease with melted vegan butter.

Make the crumble topping:

• Add the flour, sugar, melted butter and cinnamon to a small bowl and mix until combined. The crumble might be a little soft but will firm up after chilling. Set aside in the freezer until needed.

Make the muffin batter:

• Optional: Add the sugar and lemon zest to a large mixing bowl. Rub together with your fingertips until fragrant.
• Add the rest of the dry ingredients to the bowl and mix until well combined. Add all the wet ingredients (except the blueberries) and mix until just combined. Add all the blueberries and gently fold through.

Assembling and baking the muffins:

• Divide batter between the muffin cups, filling each one close to the top of the pan.
• Optional: Dollop around 2 teaspoons of the blueberry preserves on each muffin and smear/spread the jam on the surface. Sprinkle the crumble on top of each muffin.
• Bake the muffins at 220°C (425°F) for 5 minutes. Keeping the muffins in the oven, reduce the temperature to 180°C (350°F) and bake for another 28-32 minutes. The muffins are ready when the tops are lightly golden brown and the crumble doesn't look wet. If you insert a toothpick in the center, it shouldn't have any wet batter (jammy bits or moist crumbs are fine, note 3). If you omit the jam topping or the crumble, bake the muffins for 5 minutes less.
• Cool the muffins in the pan for 10 minutes then transfer to a wire rack.
• Enjoy the muffins warm or at room temperature! Store the muffins in an airtight container at room temperature for 2 days, in the fridge for 3 days or in the freezer up to 1 month. The crumble topping will soften after 1 day.