My recipe is based on what's written on the can of Off Tempo milkshake IPA...
I've drunk a fair few different ones and while I can't say I'm much of a fan of the style I know how I'd put one together. It can be simplified based on what you've got to hand .. basically ..
Produce a proteinous wort designed for average attenuation. Colour can be pale or darker, up to a deep orange, but true caramel/toffee/biscuit notes would be distracting from the hops. You can do a beer like this with lots of different malts, but the paler and more neutral the better because the more other malts you can use for impact before going into orange territory. Using some vienna and caramalt to give the same colour as a much smaller amount of light crystal gives a very different flavour. Oats (or wheat) isn't optional, but the lack of copper fining will be what makes it look like milk.
Water should be low in residual alkalinity and accentuate softness, fullness and sweetness with a chloride bias, the profile of a modern juicy IPA rather than the traditional burton snatch. The rA is for mouthfeel and subsequent impact upon bitterness which is mainly down to pH at all stages of production. When you aren't using a few bags of crystal malt alongside best ale malt rA cannot be left at 60 and still get a good mash pH. This carries over into the boil and impacts upon bittering character. This is standard in these hazy tropical IPA's.
Lactose should be used to make the beer heavier in body to bring up the sweetness and promote a creaminess, but NOT excessive, into the realm of ridiculousness. Average attenuation keeps helps keep this sensible. Too much lactose and dextrines will negatively impact upon drinkability for an IPA and you do not want to have to use a lot of IBU's to balance.
Late hops and substantial dry hopping, should be fruity, bright, tropical, juicy etc.
Beer should have adequate settlement time to minimise the potential for hop burn from the high dry hopping rates which would ruin the 'milkshake' effect.
Yeast should be average in everything, neutral.
Also would say that fruit puree/concentrate wouldn't be unwelcome, neither would be vanilla, neither would I be surprised to detect some artificial 'enhancements' in some of them.