1. Begin by melting the butter in a small saucepan over medium heat. Stir it occasionally until it turns brown and releases a nutty aroma, about 5 minutes. Be vigilant during this step—once it starts to foam, keep a close eye to prevent it from burning. Remove it from the heat immediately and transfer the melted butter to a separate bowl to cool.
2. Once the butter has cooled for about 5 minutes, stir in both the sugar and brown sugar. Mix well until it’s thoroughly combined. The sugars should dissolve nicely, giving you a smooth mixture.
3. Next, add the egg and vanilla into the bowl and mix until well combined. The batter will begin to get creamy and should look glossy.
4. Now, incorporate the mashed bananas and sour cream. Stir until fully combined. The batter should be thick and slightly lumpy, which is perfect for these cookies!
5. In a separate bowl, combine the flour, baking soda, baking powder, and salt. Whisk together to evenly distribute the dry ingredients.
6. Gently fold the dry mixture into the banana mixture. Be careful not to overmix; you want your cookies to be soft and fluffy. The dough will be slightly wet.
7. Now, fold in the mini chocolate chips and chopped walnuts. They will add amazing flavor and texture to your cookies.
8. Cover the dough with plastic wrap and refrigerate for at least 30 minutes. This step is essential as it helps the flavors meld together and makes the dough easier to handle.
9. Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Line a baking sheet with parchment paper.
10. Once the dough is chilled, use a cookie scoop or your hands to drop one inch balls of dough onto the prepared baking sheets, leaving about 2 inches of space between each. Bake in the preheated oven for 10 to 12 minutes. Keep a close eye on them; the edges should turn golden brown, but the centers will look slightly underbaked.
11. Once baked, remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack. As they sit, they will become more moist and delicious. Store them in an airtight container to keep them fresh.