Put butter in a small pot. Head over medium heat.
Allow the butter to continue cooking, stirring often as it melts, then bubbles and foams. Scrape the sides and watch for it to get a nutty smell, and start to turn to an amber color. Remove from heat.
Pour the butter into a separate bowl and let it set to cool to room temperature for about 30-45 minutes.
Chop the pecans finely. Chop the chocolate bar into bite sized pieces.
Preheat the oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper, or a silicone liner.
In a large bowl, whisk together brown butter, smashed banana, vanilla extract, and sugars until smooth.
Stir in the salt, baking soda, and cornstarch just until mixed in.
Gently stir 1 cup of flour in at a time, just until it's all mixed in.
Gently fold the pecans and chopped chocolate bar evenly into the cookie dough, until just mixed in.
Using a large cookie scoop, scoop up about 3 TBS of dough and roll the cookie dough into balls. Place 9 to 12 cookies on a sheet, about 2 inches apart.
Bake the cookies for about 10 minutes, or until the cookies are just golden brown.
Remove the cookies from the oven and let them cool for 10 minutes on the pan, before transferring them to a cooling rack. Let them cool more as desired.
The cookies can be stored in an airtight container, or ziptop bag, at room temperature for up to 3 days.