Preheat oven to 350°F (175°C). Line a 9x9-inch pan with parchment paper, leaving overhang on two sides for easy removal.

For the brownie layer, in a medium bowl, whisk together the flour, cocoa powder, cornstarch, espresso powder, and salt. Set aside.

In a separate large bowl, whisk the melted butter with the granulated sugar and brown sugar until fully combined and glossy.

Add the eggs, egg yolk, and vanilla extract. Whisk vigorously for 30–60 seconds until the mixture thickens slightly and looks smooth and shiny.

Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Do not overmix. Fold in the semisweet chocolate chips. Spread the brownie batter evenly into the prepared pan.

For the pb cookies, in a mixing bowl, use a hand mixer to beat the peanut butter and butter together until smooth and creamy. (about 30 seconds)

Add the brown sugar, granulated sugar, egg yolk, and vanilla extract. Mix until well combined and fluffy.

Add the flour, baking soda, cornstarch, and salt. Mix on low or fold gently until a soft dough forms. Do not overmix.

Drop spoonfuls of marshmallow fluff over the brownie batter. Scoop small chunks of peanut butter cookie dough and scatter them over the top like a cobbler, pressing lightly into the surface.

Bake for 32–35 minutes, or until the edges are set and the center is slightly soft but not jiggly. After baking, top with some nutter butter cookies- if desired!

Allow to cool completely in the pan before lifting out using the parchment overhang and slicing.