H. P. Lovecraft (writer).
This is the clearest case I know of where Lovecraft actually writes something suggestive of the “clap your hands if you believe” trope, a.k.a. the Tinkerbell effect, where belief itself has material consequences outside the believer’s head (and not in a dream). The eponymous character says:
He was, in fine, made sensible that all the world is but the smoke of our intellects; past the bidding of the vulgar, but by the wise to be puffed out and drawn in like any cloud of prime Virginia tobacco. What we want, we may make about us; and what we don’t want, we may sweep away. I won’t say that all this is wholly true in body, but ’tis sufficient true to furnish a very pretty spectacle now and then.
Notice the hedging. Less than a year after Lovecraft wrote “He”, he resubmitted “The Call of Cthulhu” (1926) to Weird Tales with a note to Farnsworth Wright saying “common human laws and interests and emotions have no validity or significance in the vast cosmos-at-large” (July 27, 1927). In “He”, the eponymous character’s magic is essentially illusion. In one of his visions, Lovecraft describes a future New York city with drones, warmly coloured fashion, 21st-century club music and vuvuzelas:
I saw the heavens verminous with strange flying things, and beneath them a hellish black city of giant stone terraces with impious pyramids flung savagely to the moon, and devil-lights burning from unnumbered windows. And swarming loathsomely on aërial galleries I saw the yellow, squint-eyed people of that city, robed horribly in orange and red, and dancing insanely to the pounding of fevered kettle-drums, the clatter of obscene crotala, and the maniacal moaning of muted horns whose ceaseless dirges rose and fell undulantly like the waves of an unhallowed ocean of bitumen.