In a medium bowl using a hand mixer fitted with the paddle attachment, beat the peanut butter, confectioners’ sugar, vanilla, and milk together on medium-high speed until combined. The mixture will be very thick, almost like cookie dough. Set aside.

Preheat oven to 350 degrees F.

Line a square 9x9 pan with parchment paper, leaving an overhang on the sides.

In a large microwave-safe bowl combine butter and chocolate. Microwave in 30-second intervals, stirring between each one until the mixture is melted and smooth.

Add the sugar to the hot butter mixture and whisk vigorously until combined. Allow cooling until just barely warm.

Add in eggs, yolk, and vanilla extract and whisk very well combined.

Use a rubber spatula to stir in flour, cocoa powder, and salt until just combined.

Pour half of the batter into the prepared pan and smooth out.

Bake in the preheated oven for about 20-25 minutes, or until cooked through but still slightly gooey in the center.

Let the brownies cool in the pan for 30 minutes.

Remove from pan and allow to cool to room temperature before slicing and serving.