Add the egg, egg yolks, sugar, lemon juice, and lemon zest to a bowl and whisk for 30 seconds until well combined.
Pour the mixture to a 16 oz glass canning jar. Place foil on top of the jar and cover tightly.
Place the stainless steel pot in the Instant Pot and put the trivet inside the stainless steel pot. Put the mason jar on the trive and then pour water in until it’s halfway up the side of the mason jar.
Lock the Instant Pot lid in place and close the pressure release valve. Press the "Pressure Cook" button until it's on "Normal" and press the "Pressure Level" button until it's on "High". Then press the +/- buttons until it's on "00:05" for five minutes. The Instant Pot will say "On" while it comes up to pressure and then will begin to count down.
When the Instant Pot is done, let the pressure naturally release for 10 minutes. To do this, just leave the Instant Pot alone and it will begin to count up. After 10 minutes quick release the rest of the pressure by opening the pressure release valve. Make sure to stay clear of the steam. Press "Cancel" to turn off the Instant Pot.
When all of the pressure is released and the float valve is down, open the lid.
Carefully remove the mason jar with an oven mitt or tongs. Place on a heat-proof surface and add the butter. Mix in the butter with a fork until well combined.
Allow the lemon curd cool to room temperature, then refrigerate to thicken.