Turn on the Instant Pot to 'Sauté' and using the 'Adjust' button, adjust the sauté heat level to 'Normal' and allow it to heat up for a few minutes so that the panel reads 'Hot' before adding the butter.
Add the butter to the pot and, using a spoon or spatula, move it around the bottom of the hot liner to help it melt more quickly.
Once the butter has melted and started to get hot, add the onion and cook, stirring occasionally until it becomes soft and translucent, 4 to 5 minutes. Do not allow the onions to brown. Add the garlic, stir into the onions and continue to sauté for another 1 to 2 minutes.
Add the cinnamon, nutmeg, and ginger to the pot and mix them into the onions. Allow the spices to cook in the onions until they start to release their aroma, approximately 1 minute.
Turn off the 'Sauté' function and add the vegetable stock, brown sugar, and salt then mix everything together well.
Add the pumpkin puree, just spooning it into the center of the pot. Do not mix it into the stock.
Close the pot and set the pressure release valve to 'Sealing'. Select the 'Soup' function and set the cook time to 10 minutes. Once the 10 minute cook time has completed, perform a 5 minute NPR (natural pressure release). As soon as all of the pressure has been released, open the Instant Pot. Turn off the Instant Pot and allow the soup to cool for a few minutes.
Using an immersion blender, puree all of the ingredients and then mix in the heavy whipping cream.