Add the butter to a medium saucepan and melt it over medium heat. Once melted, continue cooking, stirring or swirling the pan frequently. The butter will foam, then the foam will subside as the milk solids sink to the bottom.
After about 3-5 minutes, the milk solids will turn golden brown and the butter will smell nutty and toasted. Watch closely during this stage, as browned butter can burn quickly.
85 grams unsalted butter
Immediately pour the browned butter into a heatproof bowl, making sure to scrape in all the brown bits from the bottom of the pan. Whisk in the espresso powder while the butter is hot so it dissolves completely. Let the mixture cool for about 5 minutes, until warm but no longer hot.
2 tablespoons espresso powder
While the butter cools, mash the bananas in a large bowl with a fork until mostly smooth. Whisk in the browned butter mixture and olive oil until fully combined.
57 grams olive oil, 300 grams overripe bananas
Stir in the granulated sugar and brown sugar until well combined, then whisk in the eggs and vanilla extract. Slowly stream in the buttermilk, whisking until well combined.
200 grams granulated sugar, 50 grams brown sugar, 2 large eggs, 1 teaspoon vanilla extract, 120 grams buttermilk
Gently fold in the flour, baking powder, and salt just until a few dry streaks remain. Fold in the chocolate until just combined, being careful not to overmix.
300 grams all purpose flour, 3 teaspoon baking powder, 1 teaspoon salt, 200 grams chopped dark chocolate
Let the batter rest at room temperature while you preheat the oven to 425°F. Line a muffin tin with 6 paper liners, leaving every other cup empty to help the muffins bake evenly. Fill each liner all the way to the top using a cookie scoop, then sprinkle extra chocolate on top.
Bake the muffins on the center rack at 425°F for 5 minutes to help them rise quickly. Reduce the temperature to 350°F and continue baking for 15-18 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.