How do you get thin black line work to look hand-drawn on a cake?

Royal icing is the right call for this look, and yes it does dry rock hard, but on a cake that's actually fine as long as the cake itself doesn't flex. The line work is only a hair thick sitting on the frosting, so cracks only happen if the cake gets squeezed or shifted in transit. Box it snugly in the car and you're fine. The blobby lines you got with buttercream are just physics, it's too dense to flow through a small tip, so no amount of thinning will give you pen strokes with it. I use a #1 or #2 round tip for line work, and honestly a parchment cone with a tiny hole snipped off beats any metal tip for that sketchy hand-drawn feel. You can vary the pressure as you go, which reads as pen strokes way better than a rigid metal tip does.

I messed around with the melted chocolate drawing idea before and it kind of worked, I just spread it thin on wax paper, let it almost set, then cut shapes and pressed them on. Lines were another story though, the chocolate kept seizing up halfway through drawing and clogging the little hole. Do you mix anything into it to keep it flowy longer, or do you just work fast in small batches? Also curious if the royal icing lines can be wiped off and redone if you botch one, or once it's down it's down?

Been doing this since before parchment cones were a thing everyone knew about. My trick for the chocolate route is to stir in a spoonful of corn syrup, makes a soft ganache that stays bendy and pipes clean without seizing on you. Learned that from an old cake decorator lady at a county fair booth decades back. Also worth trying, chill the cake first so the surface is firm, then draw straight on it with the royal icing. Cold frosting gives the line something to sit on instead of sinking in. Practice your strokes on a plate first, wrist moving, not fingers.

Agree with the corn syrup trick, it saved me on a set of drawn letters that kept breaking up on me. I mix mine a touch looser than feels right, like warm honey, becuase it tightens up in the bag fast. One thing that helped me was keeping the piping bag half tucked in my palm so my hand warms it, sounds silly but the chocolate stays flowy way longer that way. Work in rows and refill often instead of one big batch.