Start by melting the 12 tablespoons of butter for the cookies. You don't want it so hot that it starts bubbling. Once it's just barely melted, place the butter in the refrigerator for 10 minutes so it slightly cools. This is an important step. Don't skip it.
While the butter cools, measure and sift together the flour, baking soda and salt in a separate mixing bowl. Set aside.
Once cooled add the melted butter along with the sugar and brown sugar to a mixing bowl and use a hand mixer or stand mixer. Blend for one minute. Start on low speed for 30 seconds then switch to medium speed for the final 30 seconds. The sugars should be fully dissolved into the butter and the mixture start to get slightly fluffy.
Add the smashed banana along with vanilla and blend just until mixed in.
With the mixer on low, slowly add the dry ingredients to the wet. Once it comes together, stop mixing. You just barely want the dry ingredients incorporated. Use a rubber scraper to scrape the bottom of the bowl and fold all the ingredients together so there are no wet or dry spots.
Using a standard ice-cream scoop, scoop 12 equal size cookies while the dough is soft. Form into balls and place on parchment paper and cover. Refrigerate for 4 hours.
For the streusel topping add brown sugar, flour, cinnamon and salt to a mixing bowl. Mix well then add 2 tablespoons of melted butter. Continue mixing. It will resemble wet sand. You want everything very well mixed. Place in the refrigerator.
Preheat oven to 350 degrees and set a rack to the middle position.
Remove cookie dough balls and streusel topping from the fridge. Using a fork, break up the streusel topping. You can also use your fingers to create small crumbles of streusel. You don't want fine granules but you also don't want huge hunks. Pea size or smaller.
Take a cookie dough ball and roll it in your hands and press the center in with your thumbs. You want to flatten it slightly and create a shallow well in the middle for the topping. The dough edges will mostly likely crack a little during this process. The consistency is like silly puddy so just press everything back together.
First roll the edge of cookie into the streusel crumbles. Once the edge has crumbles, turn upside down and plant the top of the cookie dough into the streusel crumbles. Place on a cookie sheet lined with parchment paper. Add a pinch more streusel crumbles to the center of each cookie.
Bake for 12-13 minutes in rounds on the middle rack. Only bake one sheet pan at a time and don't overcrowd the pan. The cookies should have a slightly golden edge in places but look puffy and slightly underdone in the middle at the end of baking. Remove from the oven and drop the pan on the counter one time. The slam will help flatten the cookie and create even better texture.
If you like a perfect edge, use a small bowl, turn over and cover each cookie individually. Gently swirl a few times in one direction then carefully lift the bowl up. You'll see a perfectly round cookie edge!
Let cool on the cookie sheet for 20 minutes. Transfer to a wire rack. Eat warm or at room temperature! They get chewier the longer they sit.