Adjust oven rack to the middle and heat oven to 350 degrees F. Line a 9x13-inch baking pan with foil, leaving a 3-inch overhang on all sides. Spray with nonstick cooking spray and set aside.
In a large bowl whisk the cocoa powder and boiling water together until smooth. Add the bittersweet chocolate chips and let them sit for 2 minutes, and then whisk until melted.
Whisk in the melted butter, vegetable oil, eggs, and vanilla until well blended. Add the sugar until incorporated. Add the flour and salt and whisk until combined.
Pour batter into prepared pan and smooth into an even layer. Bake 30-35 minutes, or until a toothpick comes out with just a few crumbs attached. Transfer to a wire rack until fully cooled, about 1 ½ hours.
FOR THE PEANUT BUTTER LAYER:
In a medium bowl or in the bowl of a stand mixer, beat the peanut butter, butter, and salt on medium speed until smooth and creamy, about 2 minutes.
Add the powdered sugar and mix until combined. Add the milk and vanilla and beat to combine, then beat on medium-high until completely smooth and creamy. If the mixture seems too dry or thick to spread, add milk in ½ tablespoon increments until you reach a spreadable consistency.
Use an offset spatula to spread the mixture evenly over the cooled brownies.
FOR THE CHOCOLATE GLAZE:
In a small, heat-proof bowl melt the chocolate chips and peanut butter in the microwave on 50% power for 1 minute. Mix and continue to melt on 50% power for 30 seconds. Mix and repeat until melted and completely smooth.
Pour over the peanut butter layer and spread evenly. Chill for at least 1 hour.
Use the foil overhang to lift the brownies out of the pan.
Slice into 24 squares and serve. Brownies can be stored for up to 5 days in an airtight container at room temperature.