Line the bottom of a 7 to 7 ¾-inch round pan with a round of parchment paper, if desired (see note). Lightly grease bottom and sides with cooking spray and set aside.

Combine the butter, sugar, cocoa, and salt in a medium microwave-safe bowl. Microwave for 30 second intervals, stirring in between, until the butter is melted and the mixture is smooth. Let cool slightly if needed; mixture should be warm to the touch but not hot.

Stir in the vanilla with a wooden spoon or rubber spatula. Add the eggs one at a time, stirring vigorously after each one. When the batter looks shiny, and well blended, add the flour and stir until you cannot see it any longer. Then beat batter vigorously for 40 strokes with the wooden spoon or a rubber spatula. Spread evenly in the prepared pan and smooth the top.

Add the metal trivet to the pressure cooker insert along with 1 cup of water plus 1 tablespoon of vinegar. The vinegar is optional, but will keep the insert and trivet from developing a white film over it.

Cover brownie pan completely with 2 layers of foil, spraying the inside of the layer that sits directly over the brownies to keep them from sticking. Prepare a sling for the pan by folding another two to three foot piece of aluminum foil into thirds lengthwise. 

Place the foil sling underneath the brownie pan and fold the edges together over the pan. This should allow you to easily lift the pan into the electric pressure cooker. Place the brownie pan levelly on top of the metal trivet, leaving the foil sling underneath. Fold down the sides of the foil over the pan so the lid will close.

Cook on high pressure for 45 minutes. Release pressure naturally.

Remove pan from pressure cooker using the foil sling and place on a cooling rack. Carefully remove foil so that any water residue doesn't drop onto the brownies. Cool to touch before serving or refrigerate for a couple of hours or overnight until the brownies develop the most delicious fudge texture ever (my favorite way to eat them).