In a medium bowl, combine the butter, sugars and egg with an electric hand mixer until smooth.
Add the baking soda, salt, vanilla, and flour and fold with a spatula.
Stir in the chocolate chips.
Divide the mixture in half.
Coat the bottom of two 8 1/2 inch round, aluminum foil baking pans. *see note
Press the cookie mixture evenly into the pans, making sure there are no uncovered spots.
Cover each pan with a dry paper towel and aluminum foil.
Stack the pans onto the trivet.
Pour the water into the pressure cooker pot.
Lower the pans into the pressure cooker.
Place the lid on top and turn to locked position.
Turn the steam vent to sealing.
Press the manual button and adjust the time to 25 minutes.
Place ice cream into a bowl and set in refrigerator to soften.
When the unit has finished cooking and the timer has counted to zero, carefully release the steam.
Remove the lid and take the pans out.
Remove the foil and paper towels from the pans and allow the cookies to cool completely.
Once the cookies have cooled, flip them over onto a hard surface to remove from pans.
Spread the softened ice cream onto one of the cookies.
Press the other cookie on top of the ice cream.
Place the ice cream cookie sandwich on a platter and freeze until the ice cream has hardened. (about 2 hours)
Slice into individual servings and enjoy!