Because the smoothie is a Work In Progress, this isn't going to be so much a recipe as a springboard for inspiration. I am still playing with what quantity of zucchini makes for the ideal flavor and texture (and won't overwork your blender), so I will update as I get close to the perfect answer.
UPDATE: It looks like just over a pound is a good amount for a smoothie (1.25, 1.33, somewhere in there). Any more will challenge your blender (and possibly your innards). This cooks down to about 12 ounces by weight; fresh zucchini loses about 40% of its original weight via evaporation. If you choose a higher amount, make sure to thaw the cubes for at least 10-15 minutes. I've been cooking up 4-5 pounds at a time, then freezing it in silicon ice cube trays, each of which makes 9 cubes of 1 fluid oz. each. 9 cubes seems to be an ideal quantity for a smoothie.
The basic recipe tastes like the batter for zucchini bread, and the chocolate one tastes like a Chocolate Zucchini Spice Cake batter.
The Olive Nation caramel flavor is a topic deserving of its own blog post, but for now I'll just say that while you could probably get away with not using it, it seems to be an essential element to recreating that cake batter flavor. It is well worth the price, but if you don't want to buy it, maybe just use a little extra vanilla extract. I have basically been signing my recent paychecks over to https://www.olivenation.com/
and I am not sorry! I receive no compensation from them; I am merely obsessed.
NOTE: As with most zucchini-forward recipes, this one will clean you out pretty good. Plan the next day's bathroom proximity accordingly.
The Basic Recipe
1-2 pounds (unpeeled, unless the peel is exceptionally tough) zucchini, cut into chunks; the smaller the chunks, the quicker it cooks1 TBSP coconut oil
8-10 ounces milk of choice
1 scoop stevia sweetened vanilla protein powder of choice*
1 tsp vanilla extract
40 drops Olive Nation caramel extract
1/2-1 oz peeled, fresh ginger, cut into chunks+
1/16-1/8 tsp cayenne pepper+
1 tsp. pumpkin pie spice
On medium heat, melt coconut oil in a saucepan that is big enough to hold your desired quantity of zucchini and allows you to stir it/toss it around. Add zucchini and cook, covered, stirring/tossing occasionally so it doesn't scorch. At a certain point, it will give off liquid, making it less likely to scorch. Cook until soft and somewhat translucent, about 15-30 minutes. Remove cover and allow some of the liquid to cook down. The amount of time depends on how concentrated you want it, and how much you want your kitchen to steam up. I often cook it an additional 30 minutes, keeping an eye on it and stirring occasionally.
Allow to cool a bit. With an immersion blender, thoroughly puree the zucchini. Cool completely, then freeze (preferably) in silicon ice cube trays or in a ziploc freezer bag.
When zucchini is frozen, and you are ready to make your smoothie, remove from freezer and allow to thaw for about 10 minutes.
Add milk to blender. Add remaining ingredients. Covered, give it a quick whirl to combine. Add half of the zucchini cubes and blend on high for about 30 seconds. Add remaining cubes and blend until smooth. Consume, and praise the heavens.
Variations
Chocolate
Add 4 TBSP cocoa powder with the protein powder, and if you have some sort of liquid chai spice add a squirt of that, or a pinch of cardamom.Aim for the higher end of the ginger and cayenne range
Orange Creamsicle
To the milk, protein powder and zucchini cubes, add only 1 1/2 tsp vanilla extract and 10 drops orange essential oil (none of the other flavorings or spices). This is also good with 20-40 drops of mango extract.
Your own variations:
Add anything that would taste good in a pumpkin pie shake (banana, Olive Nation banana extract, coffee flavor or instant espresso powder, etc.) or in zucchini bread (blueberries or Olive Nation blueberry extract, whatever you can come up with!). I've been adding fenugreek powder for a maple flavor, and 10-20 drops of Liquid Smoke, which adds an additional depth that you can't quite place. I was going for a zucchini-blueberry-pancakes-cooked-on-a-griddle-next-to-bacon flavor :)*If you only have unflavored and/or unsweetened protein powder, add extra vanilla and stevia to taste
+ I like mine on the zesty side. This range works well with plant protein, but if you are using whey or hydro-beef based protein (both of which have a more refined flavor, less like cake batter and more like ice cream), it might be too much.