Pumpkin Banana Muffins with a Pumpkin Spice Streusel Topping

Begin by making the streusel topping. Melt butter and add it to a small bowl along with flour, brown sugar and pumpkin spice. Mix well until crumbly and a grainy consistency. Place in the fridge to firm up while you make the muffins.
In a medium bowl, combine the wet ingredients; pumpkin puree, smashed banana, eggs, melted butter, yogurt and extract if using. Whisk together till well combined.
In a separate large mixing bowl, whisk together the flour, sugar and baking powder. With a rubber spatula, gently fold the wet ingredients into the dry ingredients. Take care not to overmix the muffin mixture.
Grease a standard muffin pan with cooking spray or butter, or line with muffin liners. Spoon in the muffin batter at least ¾ full, or all the way to the top. Filling up the muffin cups will make 9-10 larger, medium-sized muffins. Filling the cups only about half way full will make an even dozen.
Now remove the streusel topping from the fridge, and get it all nice and crumbly with your fingers. Sprinkle generously atop each muffin, concentrating in the center and piling it up.
Bake at 350 degrees f for 30 minutes. Check for doneness by inserting center of muffin with a toothpick or butter knife.
Allow to cool for a few minutes, then remove from muffin tin and place on a wire rack to cool completely.