Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large mixing bowl, whisk together flour, sugar, baking powder, and salt.
In another bowl, combine melted butter, milk, and eggs. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the mixed berries and nuts if using.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 20-25 minutes, or until a toothpick comes out clean.
Remove from the oven and let the muffins cool in the pan for 5 minutes.
Transfer muffins to a wire rack to cool completely.
Enjoy fresh or store for later!