Combine all ingredients (including the date soaking liquid) in a blender or food processor. Taste. Add water to achieve desired consistency. Transfer to a bowl or storage container.
For the rolls:
Slice mushrooms and place in a dish with a flat bottom (like a baking dish). Coat lightly with soy sauce and let marinate while you are preparing the other ingredients.
Julienne carrots and cucumbers. Wash collard greens. Cut out stems of collard greens to make uniform wrappers (2 wrappers per large collard leaf). Break off ends of cilantro stems and/or chop cilantro. Slice avocado thinly, lengthwise.
Lay collard wrapper on a clean surface. Arrange a few each of above ingredients (just 1 portabella mushroom slice or 3-4 baby bella slices) in the middle of each collard green leaf and roll. Place spring rolls, open side down, on a plate. Serve with dipping sauce.