Instant Pot® Bahn Flan (Vietnamese Flan)

Warm milk in a saucepan over medium heat. Add 2/3 cup sugar and stir to dissolve, about 3 minutes. Make sure milk does not boil; turn off heat if necessary.

Whisk eggs and egg yolk by hand; do not overmix. Pour slowly into the warm milk-sugar mixture and stir continuously over low heat for 1 1/2 minutes.

Strain through a tightly woven mesh strainer into a bowl to remove any solids that may have formed. Add vanilla extract to the bowl; mix until well combined. Set aside.

Pour 1 cup sugar into a wide skillet over medium heat. Stir occasionally with a wooden spoon until sugar has evenly melted and turned a golden caramel color, about 5 minutes. Immediately remove from heat and pour into a pressure cooker-safe cake pan, coating the bottom with liquid sugar. Pour in custard. Cover tightly with aluminum foil.

Place a trivet in the bottom of a multi-functional pressure cooker (such as Instant Pot®) and add 1 cup water. Place the cake pan onto the trivet and close and lock the lid. Seal the valve and select high pressure according to manufacturer's instructions; set timer for 9 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer's instructions, for 10 minutes, then turn the valve to Venting to release remaining pressure. Unlock and remove the lid. Remove the trivet and pan.

Allow to cool for 1 hour, then chill for at least 3 to 4 hours, or overnight. Slide knife carefully around the edges and hold the bottom securely as you invert onto a large serving dish; the caramel should drip over the flan and onto the plate.