Remove the skin and seeds from the pumpkin and cut it into small pieces.
Place the pumpkin in a steamer and steam for 15 minutes.
Transfer the steamed pumpkin to a bowl and mash it until smooth.
Add a little milk and mayonnaise to the mashed pumpkin and mix evenly.
Chill the mousse in the refrigerator for 30 minutes.
Serve chilled on a plate.