Preheat your oven to 350°F (177°C). Grease an 8x4" loaf pan with butter and line it with parchment paper.
Brown the 70 g butter (5 tbsp) in a saucepan over medium heat, swirling and scraping the bottom occasionally, until it smells nutty and turns golden brown. Pour into a small bowl to cool slightly.
Mash 2 bananas in a large mixing bowl.
Mix in the 1 egg, 80 g sour cream (1/3 cup), 80 g milk (1/3 cup), browned butter, 7 g extra virgin olive oil (1/2 tbsp), 90 g brown sugar (1/2 cup), and 4 g vanilla extract (1 tsp). Mix until smooth and well combined.
Add dry ingredients: sprinkle in the flours (134 g all purpose flour (1 1/8 cup) and 26 g whole wheat flour (1/4 cup)), 7 g baking soda (1 tsp), and 1.5 g sea salt (1/4 tsp). Stir gently until just combined—don’t overmix.
Fold in the 110 g chocolate chips (3/4 cup).
Pour into pan, smooth top, and score a shallow line down the center with a knife—this encourages a nice, even split as it bakes. Bake for about 45–50 minutes, or until a toothpick inserted in the center comes out clean.
Cool in pan for 20 minutes, then lift out and finish cooling on a rack.