Combine milk and lemon zest in a small saucepan over medium-high heat. Bring to a boil, whisking frequently.
Once milk has become frothy, remove from heat and allow to cool for 15 minutes without stirring.
Remove any skin that has formed on the top and discard, then use a fine mesh strainer to remove solids.
In a medium bowl, beat together eggs, sugar and lemon extract until sugar has dissolved.
Pour milk mixture into egg mixture a little at a time, stirring constantly. Stir until smooth and creamy.
Coat the inside of a Silikong Egg-Bite Mold with nonstick spray. Pour batter evenly into the Silikong mold and cover with foil.
Pour one cup of water in the Instant Pot and insert the steam rack. Carefully lower the egg bite mold on to the steam rack.
Secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOK function*. Use the +/- keys and program the Instant Pot for 6 minutes.
When the time is up, quick-release the pressure.
Carefully remove the Silikong egg mold from the pot. Remove foil and gently press one Golden Oreo onto the top of each custard.
Replace foil and allow to cool to room temperature, then refrigerate for at least 2 hours.
Remove from mold and garnish with powdered sugar and additional lemon zest (optional).