Grease four ramekins with butter and set them aside.
In a medium microwave-safe bowl, combine butter and chocolate chips and heat in 30-second intervals until melted. Make sure to stir well after each interval, as the residual heat will melt the chocolate.
Add powdered sugar, eggs, egg yolks, and vanilla to the melted chocolate and whisk until smooth.
Add cocoa powder, espresso, flour, and salt to the mixture and whisk until just combined.
Fill ramekins halfway with batter, then add to each a heaping 1 tablespoon of peanut butter.
Top with remaining batter, leaving about 1 inch for the cakes to expand.
Cover ramekins tightly with foil.
Place trivet inside Instant Pot and pour in the water.
Arrange three ramekins onto the trivet and stack the fourth one in the center on top.
Secure lid and seal the valve on old models, Pressure Cook/Manual on High for 20 minutes.
After that, follow the manufacturer’s instructions to do a quick release.
Carefully open the lid and using tongs remove ramekins from the Instant Pot.
Uncover them and run a butter knife around the edges. After that, invert each ramekin onto a plate, then dust with powdered sugar before serving.