hi
i would like to know is it best to make a birthday cake in layers or just in the one tin when i make it in one tin it tends to sink in middle !!

Is this a trick question?
(if you know what doesn't work, then do the other.)

if it's sinking that usually means it's not done and needs to cook longer (there are other things that can cause it, but this is the most common)

What size/depth cake pan are you using and how long are you baking?

approx 10 -12 inches and about 4 inches high i bake it for about 1 hour or a bit over