Cranks and chain slip when applying torque pedalling hard

The only reason(s) for slipping is a worn drive train.

It isn't the chain or the cassette which you say are new. Box fresh new or 'new to me' bought from a bloke with a wide suit on, pencil moustache large range of watches and ladies nylons? Any doubt just compare teeth sizes of top 3 and bottom 3 with cogs in the middle (which certainly at my house are the least used)

Which leaves chain rings, jockey wheels and freehub.

Jockey wheels will make your drive train jump and skip, not slip. Jockey wheels merely provide chain tension, If they were completely fubar'd the mech arm would invert allowing chain to simply pull through.

Which logically leaves chain rings and freehub.

Freehub test - you'll need a reliable strong friend and a cricket box. friend holds back wheel you stand on the pedals applying forward torque. If it is the freehub you'll need the cricket box to stop stem/knacker interface disaster. Also expect to lose skin (both of you). Alternatively freehubs are dead dead easy to service, just awkward because everything pings off and ends up under the frudge. Shimano freehubs are cheap as chips too.

I'd still argue it's your chainrings. The only teeth visible on the middle ring - one tooth certainly looks worn. Outer ring looks worn too. Easy to check simply compare to a new chain ring.

Very hard to work out what it is as you can't observe what is going on when it happens. When it happened to me I was certain it was rear end related (cassette or freehub) as wehn I looked down the chain was always engaged with the chainrings. Process of elimination. Then kicked myself as it was obvious once I'd worked out what was happening.

With regard to the passionate arguments about lubrication, yes will affect smoothness of shifting and rotation it wont affect a new chain in the manner described.

[style]new[/pirate mode]It's chain rings I tell ya, chain rings[/end pirate mode, ahhh Jim Lad]

Will reply in depth later but it cannot be the chain rings as they were bought new from CrC and haven't been ridden

My money is back on the number of zip ties on the stays.

This is a conundrum of endless permutations, with either zen or insanity looming just beyond the horizon.
Had this sort of problem a year back and went through all the possibilities, mentioned on here, fixing it – had a new-ish SRAM 7speed, chain and ring on which kept slipping, etc, particularly under pressure on middle cogs. Would also strangely feel like pedalling in butter sometimes. Took the cassette off and noticed a pin that held the stack together was dropping out and a few teeth on several middle cogs were slightly out of alignment.
Fitted a new freehub and popped on my old slightly rusty Shimano 7speed from the parts bin (20 yrs old). Runs like a dream.

Not going to believe this, but now I have the exact same problem! I have a brand new SRAM 9-speed chain, one year old SRAM cassette which I feel is unlikely to be worn, brand new middle chainring and cassette is running on a 1 year old hope pro2 hub.
First things I am going to do are: fit new jockeys as mech is 3 years old; fit replacement derailleur hanger; remove cassette and check for any wobbly sprockets; check rear derailleur - especially cable tension.

Man, I'm at a loss as how to solve it
Cassette is new as is chain and rear me h is in tip top condition.
I might change te cassette to a different ratio see what happens

Did you get your derailleur hanger checked?

not sure if anyone has mentioned this but could it be a stiff link in the chain?

Check the B-tension screw to make sure you have enough wrap round on the cogs, chain ideally needs to contact at least 50% of the cog

Thanks for all the help and advice
Time to test now according to the responses