Weigh out the ingredients
You'll need flour, sugar, brown sugar, vanilla, butter, baking powder, salt, bananas, eggs, and Nutella.
Combine the dry ingredients
In a medium bowl, combine the flour, baking powder, and salt. Set aside.
Melt the butter over medium heat, stirring with a rubber spatula as it melts. Once the butter turns a dark golden color and the fat solids have darkened and settled at the bottom of the pan, remove from heat and immediately pour into a large bowl, scraping out the fat solids at the bottom. Weigh out the butter to 184g (13 tbsp).
Brown the butter
To the brown butter, whisk in the brown sugar and granulated sugar. Then, vigorously whisk in the egg, egg yolk, and vanilla until fully combined.
Mix in sugars, eggs, & vanilla
On a medium plate, mash the bananas until smooth.
Whisk the mashed bananas into the batter.
Mix in the bananas
Add the dry ingredients to the batter and use a rubber spatula to fold until just combined.
Mix in the dry ingredients
Prepare a 9 inch baking pan with parchment paper. Pour the batter into the pan, then dollop the Nutella on top and use a toothpick to swirl it in.
Swirl in the Nutella
Bake the banana blondies for about 30 minutes until evenly golden. Allow them to cool completely in the pan before removing and cutting into 16 squares.
Bake & enjoy!