Gather the ingredients.
Preheat the oven to 350 Fand grease a 9 x 9-inch pan with cooking spray or softened butter. Line the bottom with a piece of parchment paper.
Place the butter and sugars in a large mixing bowl and whisk until fully incorporated.
Add the eggs and yolk one at a time, whisking gently after each addition. Add the vanilla and gently whisk again.
Add the miso and whisk until smooth.
Whisk the flour, baking powder and salt together in a medium sized bowl. Add the chunks to the bowl and whisk again.
Add the dry ingredients to the wet and using a rubber spatula, very gently fold the two together. Do not over-mix.
Transfer the batter to the prepared pan and use a flexible spatula to smooth the top.
Sprinkle the Maldon sea salt (if using) over the top of the blondies.
Bake for about 35 to 38 minutes, until the top is browned and a toothpick comes out with a moist crumb or two.
Let the blondies cool on a rack until room temp and slice and serve.