Round cake in square tins!

I am frequently asked to bake cakes for occasions where 1 slice will be served per person. A square cake would be easier to cut and serve so how do I amend the recipe for instance from a 20cm round (Blueberry & Pecan muffin cake star of the moment!).

Can you change a tart from round to square too?
Thank you for making me look good but baking is so easy now! The benefits of retirement.
regards Linda