If my recipe says....how do I....?

[HuntedForest]

Cake recipe says put in a 23 cm round baking tray and bake for 45 minutes.

How long do I need to cook it if I use half the mix in a square 15 cm tray?

What kind of cake is it? Sponge?

Chocolate cake

Very dense? Or more airy like a sponge? I would go for 20 minutes to start with and then check it every 5 minutes. If it's a sponge it will probably be done after 20 minutes.

It's quite dense, I think, there's no baking powder or bicarb in the recipe.

The area of the round tin is about twice the area of the square tin so the depth of the mixture will be the same if you use half the mixture in the square tin, so the middle of the cake that takes longest to cook because it is furthest from the heat is in approx the same position. I would guess it will take alost as long to cook but I'd be checking after 20 minutes. You can always cook for longer if it fails the skewer test but you can't unburn a cake

You could try this.