Preheat oven to 200°C.
Take a large bowl. Add the flour, sugar and baking powder. Mix well.
Take 12 Oreo Cookies and chop them.
Take a medium bowl. Add egg, milk, butter and vanilla extract. Whisk until well-combined.
Stir in the mashed bananas.
Add the wet mixture to the dry mixture. Stir just until it turns moist.
Stir in the chopped Oreo Cookies.
Take the remaining cookies. Split them in the middle. Scrape off the creme from the cookie. Transfer the creme to a microwaveable bowl and set aside. Chop the cookies.
Take a 12-cup muffin pan. Line the cups with paper.
Spoon the batter into the muffin cups. Sprinkle the chopped Oreo Cookies. Gently press the cookie crumbs into the batter to make it firm.
Bake for 15 to 20 minutes or until a toothpick inserted into the centre of the muffin comes out clean. Cool for five minutes.
Remove the muffins from the pan and place them on a wire rack. Cool for 15 minutes.
Heat the reserved creme in the microwave on high power for 15 to 30 seconds or until it melts. Stir the creme.
Drizzle it over the muffins. Oreo "Streusel" Banana Muffins are ready! Serve warm.