Heat oven to 425°F (218°C) and prepare a 12-cup muffin tin with liners or a light spray. Starting with high heat creates beautifully tall muffin tops.
Combine streusel ingredients in a small bowl until crumbly and set aside. This adds a crunchy, sweet topping to your muffins.
Mix all-purpose flour, baking powder, baking soda, cinnamon, and salt in a medium bowl. This ensures the even distribution of dry components.
Cream butter and sugar in a large bowl until fluffy. Beat in the egg, then add mashed bananas and yogurt, mixing until slightly lumpy.
Gradually add dry ingredients to the wet mixture at a low speed. The muffin batter will be thick, which is perfect for fluffy muffins.
Layer batter and streusel in muffin cups, starting and ending with streusel. Use a toothpick or knife to gently swirl the topping into the batter.
Bake at 425°F for 5 minutes, then reduce heat to 350°F and bake for 15–16 more minutes. Check doneness with a toothpick.
Cool muffins for 5 minutes in the pan, then transfer to a rack. Enjoy!