Preheat oven to 425 degrees F. Lightly coat a muffin tin with baking spray or line with paper cups. (This recipe usually yields 12 regular-sized muffins for me.)
In a large bowl, mash the bananas with a spatula or fork until they are soft and mostly smooth. Add the egg, melted butter, yogurt, milk, and vanilla extract; stir to combine. Add sugar and whisk until the mixture is smooth.
Sprinkle the flour, baking powder, and salt on top of the banana mixture. Fold gently with a spatula just until the flour is mostly incorporated.
Spoon 1-2 Tablespoons of the batter into the bottom of each muffin well. (This ensures that the strawberries don't all sink and stick to the bottom.)
Add most of the strawberries to the remaining batter, then fold gently to incorporate. Divide batter among the 12 muffin cups, topping each one to roughly 3/4 full. Place 2-3 sliced strawberries on top of each muffin, if desired. (This is only for looks.)
Bake at 425 degrees for 5 minutes, then, without opening the oven door, reduce the temperature to 350 degrees and bake for another 12-15 minutes. When done, the muffins will be lightly golden on top and a toothpick inserted into the centers will come out with moist crumbs, but no large streaks of batter.
Let cool for 10-15 minutes in the pan, then remove to a wire rack to cool completely.
Notes
You can make these with either very ripe bananas or with bananas that have been frozen whole and then defrosted. I've done both and had great results each way.
I have actually had luck ditching the paper liners and simply coating the inside of my wells well with cooking spray. Less waste and, surprisingly, less sticking overall.