One of the first employees of $3.5 billion startup Stripe is leaving to form his own company

Stripe's fourth employee, CTO Greg Brockman, announced today that he will be leaving the payments processor Friday.

Stripe is one of San Francisco's hottest startups, with a valuation of $3.5 billion as of last December. The company makes it super-easy for web site owners and app developers to accept payments via credit card and other systems, including Apple Pay.

"About six months ago, I found myself in a strangely luxurious place: I'd successfully removed myself as a critical lynchpin for the company and could craft literally any role I wanted," Brockman said in his blog post detailing his departure.

"But the more that I explored, the more I realized: I was yearning to create something of my own."

Brockman joined Stripe when it was still called "/dev/payments" and left school at MIT to join Stripe's founders Patrick and John Collison and their third mate Darragh Buckley.

While at Stripe, Brockman officially took on the role of CTO in early 2013, but it took him about a year and a half to define what that role was. Brockman returned to writing code to get a better perspective on the organization.

Brockman publicly backed Stripe's investment in cryptocurrency nonprofit, Stellar, last July, although Stellar has yet to gain the same momentum or growth as other cryptocurrencies like Bitcoin or money transfer protocols like Ripple.

Six months ago, though, Brockman said he'd successfully removed himself from any critical decision-making so now he sees it as time to move on. Brockman told Business Insider that he has nothing to share about his plans just yet.

"Right now I'm in a rare window. I haven't yet placed myself in a new critical role, and I'm confident that Stripe is being left in great hands," Brockman said in his note. "If I don't take advantage of this opportunity, I'll always wonder what could have been."