In a medium-sized saucepot, bring milk, sugar, cornstarch, salt, lemon zest and salt to a boil, stirring constantly.
Once the mixture comes to a boil, remove from heat.
Combine 2 eggs and 2 egg yolks in a mixing bowl. Slowly add the hot lemon custard mixture a little at a time to the egg mixture, whisking constantly. This will prevent the eggs from cooking.
Whisk in butter until melted, then add lemon juice. Stir until smooth and creamy.
Pour equally into 4 8-ounce oven-safe ramekins, leaving about 1/2" space at the top. Cover each ramekin with aluminum foil.
Place trivet in Instant Pot and add 2 cups water.
Carefully place ramekins in the Instant Pot (3 on the bottom forming a triangle and 1 over the top in the center).
Place lid on Instant Pot and set tab to "sealing". Set to "manual" and set to "high pressure" for 5 minutes.
When the time is up, do a quick release and turn Instant Pot off. Allow ramekins to cool to room temperature and then place in refrigerator for a minimum of 2 hours.