Melt butter in a small saucepan over high heat, whisking constantly (do.not.step.away!). After about 3–5 minutes, it’ll turn golden, smell nutty, and develop brown bits at the bottom. Immediately remove from heat and let cool.

In a large bowl, whisk together browned butter and brown sugar until well combined.

Add eggs, mashed banana, sour cream (or Greek yogurt), and vanilla. Whisk until smooth.

In another bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.

Gently fold the dry ingredients into the wet mixture, just until no dry streaks remain. Fold in the walnuts.

Cover and let the batter rest for 30–60 minutes at room temp, or even better, overnight in the fridge. This hydrates the flour and gives you tall, bakery-style muffin tops. (Yes, it really makes a difference.)

Preheat oven to 425°F (218°C). Line or grease your muffin tin and fill each cup all the way to the top. Top with extra walnuts, banana slices, and/or turbinado sugar.

If you like a denser, cake-textured muffin: bake at 425°F for 13-15 minutes for standard size muffins, or 17-20 minutes for jumbo muffins.

If you like a more spongey, light muffin: bake at 425°F for 5 minutes, then reduce to 350°F (177°C) and continue baking — jumbo muffins 18–22 more minutes, standard muffins 14–16 more minutes.

They’re done when a toothpick inserted in the center comes out clean.

Let the muffins cool in the tin for 5–10 minutes, then transfer to a rack to cool completely (if you can wait).