Preheat the oven to 375F and grease or line a muffin pan with paper liners.
Make the batter:
In a medium bowl, combine the flour, baking powder, and salt.
In the bowl of a stand mixer or a large bowl using a hand mixer, beat the butter and sugar for about 2 minutes, until it's light and fluffy. Add the vanilla and eggs, one at a time, mixing them into the sugary butter mixture well before adding the next one.
At low speed and gradually, add the dry mixture, alternating with the milk, into the bowl. Once combined, stop.
Add the frozen mixed berries into the batter bowl. With a spatula, fold them in to combine.
Scoop the muffin batter into the prepared muffin pan, nearly to the top, and sprinkle the tops with sugar.
Bake the muffins:
Bake the muffins for 27 to 30 minutes, until the tops are golden and a toothpick inserted in the middle comes out clean. Remove from the oven and let them cool down in the pan for 10 minutes. Remove from the pan and let them cool down completely on a rack.
Store:
Store leftover muffins in an airtight container in the fridge for up to a week or freeze in a zip bag for up to 3 months. Thaw them on the counter or lightly microwave them for 60 seconds.