Preheat the oven to 375°F and line a 12-cup muffin pan with paper liners so the muffins release cleanly and bake evenly from the start.
Whisk together the flour, baking powder, baking soda, salt, and sugar until evenly combined because this helps distribute the leavening agents throughout the batter.
In another bowl, mix the eggs, melted butter, milk, vanilla extract, and Greek yogurt until smooth and creamy to create a moist muffin base.
Stir the wet ingredients into the dry ingredients gently until just combined, then carefully fold in the mixed berries without crushing them.
Divide the batter evenly into the muffin cups, top with coarse sugar if desired, and bake for 20 to 22 minutes until golden and fluffy.
Let the muffins cool in the pan briefly before transferring them to a wire rack so they hold their shape and texture properly.