In a microwave safe medium bowl, melt the butter with the chocolate (or do this in a small saucepan over low heat). Meanwhile, butter four 6-ounce ramekins.
To the bowl with the butter/chocolate mixture, add the confectioners’ sugar and salt and whisk until cool to the touch. Whisk in the whole eggs and egg yolk, followed by the bourbon and flour. Distribute the mixture evenly among the four ramekins. Cover each with aluminum foil.
Pour 1 cup water into the pressure cooker pot. Place a steamer rack/trivet at the bottom of the pot. Stack the ramekins on the rack. You should be able to fit three on the bottom with the fourth placed on top in the center.
Lock the lid into place and cook on high pressure for 9 minutes. Manually release the pressure.
With tongs, remove the ramekins from the pot and allow them to sit for 3 minutes, still covered with foil. Then remove the foil and run a knife around the edge of each cake. Flip each cake onto a serving plate. Sprinkle with sea salt and top with crème fraîche or ice cream, if you like. Serve immediately.