Preheat the oven to 325°F.

Line an 8x8-inch baking pan with parchment paper, leaving enough overhang to lift the blondies out after baking.

Line a small plate or tray with parchment paper.

Using two spoons, portion the marshmallow fluff into 9 equal dollops.

Place in the freezer while preparing the batter.

In a large mixing bowl, use a hand mixer on medium speed to beat together the melted butter, brown sugar, and granulated sugar until smooth and glossy.

Beat in the peanut butter until fully incorporated.

Then add the eggs, egg yolk, and vanilla bean paste and beat again for about 1–2 minutes until the mixture becomes slightly lighter and smooth.

Add in the flour and salt. Mix just until no dry streaks remain. Do not overmix.

Divide the batter, setting aside about one-third of the blondie batter.

To the remaining two-thirds of the batter, beat in the cocoa powder and melted dark chocolate until fully incorporated.

Fold in the chopped chocolate.

Spread about two-thirds of the chocolate batter evenly into the prepared pan.

Arrange the whole Nutter Butter cookies in an even layer.

Then spoon the reserved peanut butter blondie batter onto the cookies and spread evenly.

Remove the marshmallow fluff from the freezer and gently press the frozen dollops into the blondie layer.

Spread the remaining chocolate batter over the top.

Finish with the crushed Nutter Butter cookies.

Bake for 32–38 minutes, or until the edges are set and the center still looks slightly underbaked.

A toothpick inserted into the center should come out with moist crumbs, not wet batter.

Sprinkle with flaky sea salt and allow the blondies to cool completely in the pan for at least 1–2 hours.