Add the sugar, a pinch of salt, and 1 cup of milk in the pressure cooker. Melt the sugar by using the slow cook function or by using low heat on the stovetop. Stir until sugar is fully melted. Remove from the heat and let the milk mixture cool down a little.
Pour in 1/2 cup of milk and mix well. The sweet milk mixture should be cool to the touch at this point.
In a glass bowl, whisk 3 large eggs until well blended. While whisking, slowly add the sweet milk mixture.
Pour the egg mixture through a fine mesh strainer two times to remove any lumps and to make smooth.
Pour filtered egg mixture into 4 ramekins. Remove any air bubbles with a spoon. Sprinkle with a light dusting of cinnamon if you like. We prefer the custard without. Wrap the ramekins really tight with aluminum foil.
Pour 1 cup of cold water into the Instant Pot. Place the trivet inside the Instant Pot, then carefully place the 4 ramekins on the trivet. You can stack them in two layers.
Close the lid and cook at LOW Pressure for 0 minutes (1 minute if you are at sea level) plus 10 minutes of Natural Release. Don't open the lid until the full 10 minutes is over. Open carefully. Set aside to avoid overcooking.
This dessert is best served warm or hot but is good cold as well.