[ywnico]

To expand on your point a bit, it depends on the type of fluid. Newtonian fluids [1] (a good approximation for water and many other "normal" fluids) will be pulled into the groove regardless of how viscous they are. E.g., the cup would probably work for treacle (syrup). Custard is non-Newtonian; more specifically, it's probably a Bingham fluid [2], meaning it acts like a solid until enough shear stress is applied, and then it acts like a liquid. The problem here is that even if there's enough shear stress for the custard far from the walls to move, the custard deep in the corner will likely remain a solid. So the corner capillary effect won't work.

[1] https://en.wikipedia.org/wiki/Newtonian_fluid

[2] https://en.wikipedia.org/wiki/Bingham_plastic