Preheat your oven to 375°F (≈190°C) and grease a 9×13-inch baking pan with non-stick cooking spray.

In a large bowl, cream together the granulated sugar and softened butter until light and fluffy.

Beat in the Greek yogurt (or sour cream), eggs, mashed bananas, and vanilla extract until well combined.

Add the flour, baking soda, and salt; mix just until the dry ingredients are moistened and incorporated. If you’re using chopped nuts or chocolate chips, stir them in now.

Spread the batter evenly into the prepared baking pan. Bake for 23–28 minutes, or until a toothpick inserted into the center comes out clean or with only a few moist crumbs.

As the bars are baking, start making the brown butter glaze: In a large skillet over medium heat, melt the butter and let it cook until it develops a light golden-brown color and a nutty aroma. Remove from heat.

Immediately whisk in the powdered sugar, vanilla extract, and 1 tablespoon of milk. If the glaze is too thick, add a little more milk; if too thin, add a bit more powdered sugar until you reach a spreadable consistency.

As soon as the bars come out of the oven (while still warm), pour the warm glaze over them and spread gently with the back of a spoon or a spatula.

Allow the bars to cool completely in the pan before cutting into squares.