In a 8oz mason jar, mix together the egg, pumpkin, molasses, sugar and milk. Mix until smooth.
Stir in spices and flours until well combined.
If using, stir in white chocolate chips.
Place trivet in Instant Pot along with two cups water.
Set jar on top of trivet and cover jar with foil or silicone lid.
Secure lid of instant pot and cook on manual for 15 minutes.
Quick release the pressure. Carefully remove jar from instant pot (it will be hot! I highly recommend using a pair or jar tongs or hot mit)
Allow cake to cool slightly, then dive in!
Notes
To make in microwave, follow the batter instructions above. Let the batter sit for 2-3 minutes. Leave the jar uncovered and microwave on high for 3 to 3.5 minutes. Allow to cool slightly, then dive in!