Simply summon a falling block as a passenger on top of another one, and it will break off into an item. To see this effect yourself, use this command:
/summon falling_block ~ ~ ~ {Time:1,Passengers:[{id:"minecraft:falling_block",Time:1,BlockState:{Name:"minecraft:command_block"}}],BlockState:{Name:"minecraft:command_block"}}
I sincerely hope this was not intentional.
And of course, as soon as I post this, I find a duplicate.
MC-145113