If you're finding yourself reaching for pen and paper to write actual code, then your tooling sucks.
Your IDE isn't suitable for prototyping, or something else is going wrong.
I'm not suggesting that nothing in the planning phase should be white-boarded, but pen is the wrong medium for code.
You should have a tool where you can fire up your tooling and be writing the same thing you would write on paper, but with autocompletion, error feedback and browsable interactive output.
In my experience that's far more appropriate an environment for rapidly getting something down and working through the problem.
By all means, start with pen and paper to get a feel for the problem domain, but that process should stop short of actual code, and this article doesn't really sell me on the benefits.
I'd rather test my assumptions in something like LinqPad, where I'll get real feedback not just a "feeling" about what I've written.