Preheat the oven to 350 degrees F and place cupcake liners in a muffin tin. Set aside.
In a small saucepan, start to melt the butter on medium/high heat. Stir with a spatula frequently, consistently watching the butter. The milk solids (specks in the melted butter) will get a nice golden brown color and will start to take on a nutty flavor.
Watch very closely because butter can go from golden to burnt in just a second. Once you see the butter start to slightly brown, remove from the heat immediately because it will still continue to brown for a minute after it's off the heat. This entire process should take about 5 minutes.
Once the butter has been browned, add the butter, sugars, and vanilla extract to a large bowl and mix until combined (you can use a hand mixer or a spatula).
Once combined, add in the two eggs and mashed banana. Miix until they are incorporated into the sugar mixture. Set the bowl aside.
In a separate bowl, whisk together flour, sea salt, and baking soda.
Add dry mixture to the wet mixture, stirring well until incorporated.
Fold in chocolate chips and pecans with a spatula.
Fill each muffin liner almost to the top (about ¾ full), then sprinkle with demerara sugar.
Bake for 20-22 minutes on the center rack of the oven. A toothpick should come out clean when inserted into the middle of a muffin.