Preheat the oven to 190°C / 170°C fan.
Line a muffin tin with 12 muffin cases.
In a large bowl, beat the eggs. Add the oil, milk and vanilla and mix thoroughly.
In another bowl, whisk together the flour, sugar, baking powder and salt.
Add the wet ingredients to the dry and gently fold together until just combined. Don't overmix.
Add the frozen blueberries and again gently fold until combined.
Evenly distribute the mixture between the 12 muffin cases. Sprinkle the extra caster sugar on top of each.
Bake for 20-25 minutes or until a skewer comes out with just a few moist crumbs.
Leave to cool in the tin for 10 minutes before transferring to a cooling rack.