"how astonishingly" is a correct and usable phrase in written English.
It can be used to emphasize surprise or amazement at something. Example: "How astonishingly beautiful the sunset was tonight!".
But "Unbuttoned" underscores just how astonishingly quick his success was.
This gives you an idea of how astonishingly small the Mac Pro is.
The success of the Star Wars franchise shows how astonishingly profitable the development can be.
As we walked past all the oblivious college students, their whole lives ahead of them, I thought about how astonishingly few people do what Everman did.
She was thinking of how the massive dog had thrown itself at her and how, astonishingly, the man had protected her.
Blair McMillen, Tatjana Rankovich and Mr. Rubenstein himself reminded us of how many astonishingly good pianists are out there working just out of the spotlight.
Her story is the stuff of classic melodrama, and that's how Greene, astonishingly, films it: his images, with their shrieking colors and vertiginous geometry, suggest the intimate grandeur and bitter irony of a Douglas Sirk romance come to life.
In "Making Sense of Life," Evelyn Fox Keller surveys efforts over more than a century to discover theories that will explain how the astonishingly varied biological forms we see in the world are generated.
There are more than 100 municipal pools in this sprawling city where Dawn Fraser and Ian Thorpe learned how to swim astonishingly fast.
It's hard to imagine she was ever so physically broken: she has the glow of someone who turned to running for therapy, takes part in triathlons, works part-time as a personal trainer, and whose Twitter bio notes, slightly astonishingly, how much she loves to swim outside.
