Preheat the oven to 400ºF/200ºC. Line a 12-hole muffin pan with paper cases.
Add the frozen berries to a microwave-safe bowl and microwave for 1 minute to thaw slightly. If any berries are very large (strawberries sometimes are) then when they’re thawed a bit cut them into halves or quarters.
In a large mixing bowl, whisk the flour, sugar, baking powder and salt.
In a smaller bowl, whisk the eggs, milk, melted butter and vanilla together.
Stir wet mixture into the dry mixture and gently mix to combine.
Spoon the berries out of any juice they’ve released (save the juice for the drizzle). Gently stir the frozen berries into the muffin mixture. Don’t overmix or the muffins will be tough and discolored.
Spoon the batter into the muffin cases to ¾ full. Bake for 20-25 mins until the frozen berry muffins are risen and golden. Allow to cool on a wire rack.
Mix the icing sugar and berry juice together to form a thick drizzling consistency, adding a little more icing sugar or juice/water if necessary. Drizzle over the completely cooled muffins.