In a high-speed blender, combine the cold coffee, 6 of the pitted dates, hemp hearts, cashew butter, and spinach, if using. Blend until the dates have broken down and the mixture is very smooth. Taste the mixture and add more dates if you'd like it a little sweeter. (I prefer using 4 dates per serving, but it is sweet!)
Add the ice cubes (I usually use a pint glass of ice cubes per serving, for easy measuring) and blend again, until a thick, frosty-like texture is achieved. It should have the texture of a milkshake. Serve right away.
Note: This smoothie has a delicious coffee flavor when you use regular ice cubes, but for an extra-potent coffee flavor, try using coffee ice cubes instead! To make them, simply pour leftover coffee into an ice cube tray and store it in the freezer overnight.