The abstract “intelligence” is so ill-defined that it’s as good as impossible to prove any intervention helps improve it.
This question is about “Intelligence as measured by IQ”, though. That’s fairly easily improved. To avoid cultural biases, many tests will include mathematical questions, and training on those can have significant impact.
For example, knowing the Fibonacci sequence and memorizing multiplication tables, squares, third powers, etc. will help speed up answering “what number comes next in this sequence?” questions (about everybody can continue the sequence “1, 2, 3, 4, 5, …”; fewer will immediately recognize “1, 8, 27, 64, 125, …” as third powers of integers, and even fewer will not have to compute 6³ because they just know it’s 216)
However, it seems that willpower can be trained and in some regards this is more useful. After all, if you're super smart but can't complete a project then that's a bit of a waste really.
Anecdotally, I would say most of the successful people I know exhibit extremely good self-discipline but are perhaps only a little bit smarter than average.
The theory is that if you are able to train yourself to be disciplined in some ways you can become disciplined in others as well, much like building strength in the gym. I think this might be the basis of the "tidy your room and keep it tidy" advice meme that seems to be repeated a lot at the moment (I should probably do that myself really)
I haven't actually but I'll check that out when there's space on my reading queue. Currently I'm working my way through "What to say when you talk to yourself" which is so far interesting but not sure if I'd recommend it yet.
In fact, there is a whole ton of research also on google scholar:
https://scholar.google.com/scholar?hl=en&as_sdt=0%2C14&q=how...