Prep the apples: peel, core and chop them just before blending — cut apple browns within minutes, and browning dulls both the colour and the flavour of the shake. Work quickly and keep the pieces ready with the blender.
Blend the base: put the apple pieces in the blender with the sugar or honey, cardamom, cinnamon and about 1/2 cup of the chilled milk. Blend until completely smooth, with no bits of apple skin or fibre left — this first thick purée is what gives the shake its body.
Add the rest: pour in the remaining milk and the ice cubes, add the vanilla ice cream now if you are using it, and blend again for 20 to 30 seconds until the shake is frothy and pourable. Do not over-blend; once the froth forms, stop.
Taste and adjust: sweet apples may need no extra sugar at all, while a tarter fruit will want another spoon. Add, blend for two seconds, and taste again.
Pour into two tall glasses at once, scatter the chopped nuts or a dusting of cinnamon over the froth, and serve immediately with a straw — this shake waits for nobody.