Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, beat the egg, then add milk, vegetable oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the mixed berries, being careful not to overmix.
Divide the batter evenly among the muffin cups.
Bake in the preheated oven for 18-20 minutes, or until a toothpick comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack.
Optionally, top with frosting or whipped cream for added sweetness.
Serve warm or store for later enjoyment.