In a heat-resistant bowl, slightly warm the peanut butter in a microwave for approximately 20-30 seconds until it reaches a pourable consistency. Ensure it remains moderately warm and not overheated.

Line an 8-inch square metal pan with parchment paper, ensuring there's excess parchment on the sides for easier extraction. Evenly distribute the warmed peanut butter across the base. Transfer the pan to the freezer and allow the peanut butter to solidify for one hour. Upon solidification, carefully lift the peanut butter sheet using the parchment overhangs and place it back in the freezer.

Preheat the oven to 350°F (177°C). Re-line the 8-inch square pan with fresh parchment paper, again ensuring overhangs for later use.

In a large, microwave-safe bowl, amalgamate the unsalted butter and finely chopped semisweet baking chocolate. Employ the microwave in 30-second intervals, mixing thoroughly after each interval, until the two ingredients meld seamlessly.

Introduce the granulated sugar into the melted butter-chocolate mixture. Whisk diligently to achieve a homogeneous blend. Set aside and allow to reach a slightly cooled temperature.

Subsequently, incorporate the eggs, additional egg yolk, and vanilla extract. Whisk for approximately one minute until fully integrated.

Gradually fold in the all-purpose flour, sifted unsweetened cocoa powder, and fine salt, taking care not to overmix.

Dispense half of the resultant brownie batter into the prepared pan, leveling it out. Gently place the previously frozen peanut butter sheet atop this layer. Cover this with the remaining brownie batter, ensuring the peanut butter layer is entirely concealed.

Place the pan in the preheated oven and bake for roughly 30 minutes. The brownies should be thoroughly cooked, retaining a slight moisture at the center. Once baked, allow the brownies to cool in the pan for half an hour. Afterward, employ the parchment overhangs to extract the brownie from the pan and let it cool further for an additional 30 minutes. Subsequent to this cooling period, the brownie can be divided into portions and served.