One Vanderbilt, which is being billed as Midtown Manhattan’s tallest office tower, has topped out its decorative twisty spire at 1,401 feet — and the views from there are spectacular.

On my exclusive visit to the tower Tuesday morning, the outside construction elevator whooshed seamlessly up the equivalent of 80 stories to where construction workers are still welding, pulling cables and tightening bolts.

I clambered up narrow metal steps for several more stories behind Edward Piccinich, the chief operating officer of developer SL Green Realty. At 1,200 feet up, while gazing out from the open concrete edges of the spire’s ironwork, we could see Billionaires’ Row towers were almost at eye level while the neighboring MetLife and Chrysler buildings were below.

I wouldn’t walk a tightrope to the Empire State Building some 10 blocks away, but there are folks who might try.

Piccinich says the $3 billion tower is three months ahead of schedule in a project that has saved $100 million due to pioneering use of state-of-the art 3-D construction modeling technology with documents that are made available on iPads. The steel is all American, from Coatesville, Pa., Lynchburg, Va., and South Plainfield, NJ and each of the 26,000 pieces was counted and barcoded before being assembled on the site into the 1.7-million-square-foot building.

TD Bank is already building out floors with everything scheduled to be fully glassed and a temporary certificate of occupancy obtained in August of 2020.

The design, by architects at Kohn Pedersen Fox, has its window washing rig and sway-cutting tuned mass damper tucked inside, leaving its skyline silhouette pristine.

Below the spire framework, a three-story observation area between the 75th and 77th floors will be open to the public.

At 732 feet up, the 73rd is the highest available office floor, with 26,770 square feet, 20-foot ceilings and sky-high asking rents to match.

The third floor will host 30,000 square feet of tenant amenity space with a giant terrace facing Grand Central Terminal, a boardroom seating 48, a 140-seat auditorium, a lounge and even showers.

It won’t be long before star chef Daniel Boulud opens a dramatic 11,000-square-foot restaurant on the second floor that will have an Épicerie Boulud grab-and-go outlet on the street level.

For commuters, the building will have a seamless path to the new LIRR East Side Access area while the same dark-cream marble will be used for its wide corridor to Grand Central. For now, Piccinich opened a still-secret door, and we said our good-byes while standing in Grand Central Terminal.