#1 cake tin - how to scale for a 6" tin?

[Pidgin]

I am making a sponge cake for my son's first birthday and want to use Nigella's birthday custard sponge from Feast. The recipe is for 2 20cm/ 8in tins, and the tin I am using (shaped like a number one) is the equivalent of a 6in tin. Would you scale it down or just put a bit extra in the tin?
And, would you use ready-roll icing for this kind of cake or stick to glace icing or something similar? Am hoping for v impressive cake but not too complicated or I will mess it up for sure.
I have made a few cakes but am sweating a bit at the challenge of the first birthday...
Thank you in advance!

I wouldn't bother scaling it down. I would just mix it up, as instructed, and then use a bit less. quite a bit less.

My mil's recipe fits a 6 inch round cake tin perfectly.
8 oz marg
8oz sugar
10 oz flour
4 eggs
tsp vanilla
pinch salt

20 mins at 200 degrees.
Simples. Tis SOOOOO easy.

Sure yours will be fine.

I would do as Oblomov says and make up whole recipe, chances are the tin you have is deeper than a standard 8 inch tin anyway, and use as much as fits in (usually to about 3/4 of way up tin for most cakes). For icing, glace will probably be too runny to stick to the sides and is likely to pool at the bottom. You can use ready roll, but if it's a Number One shape cake, chances are it has angles/corners etc and these will be tricky to fit rolled icing round. I would therefore use a stiff buttercream. Good luck!

And what cooking times does nigella give for her 8 inch tin ? You will need to scale yours down aswell. scale down the amount of time you cook it for. this is where it gets really tricky.and you could come a cropper!!
but I always think its better to undercook a sponge than overcook. it does of course carry on cooking for a bit , so will probably end up perfect.

Thanks Tiger and Oblomov! Buttercream I can do & that is a good point re cooking times, I guess I would need to bake it for 75% of the time as the tin is 75% of the size - gcse maths not a total washout after all! or I might just use your mil's recipe Oblomov, it looks simpler too. I think I might need a few practice goes anyway. Thanks again.