I want to make a smaller batch for the brownies - can I just reduce the ingredients proportionately? Or is there some other logic to follow?
Nigella's Brownies (from HOW TO BE A DOMESTIC GODDESS) is a recipe for walnut-studded brownies that are baked in a 33x23/13x9-inch rectangular pan. The Brownies will keep for up to 5 days in an airtight container or can be frozen for up to 3 months, so if you make the regular amount of Brownies then there are ways to store the leftovers.
Scaling cake recipes can be tricky and not straightforward. In this particular case we suspect that it may be possible to bake a half quantity of the Brownies in a 20x20cm/8x8-inch pan. The Brownies will be slightly thinner than for the regular recipe so you should watch them carefully while they are baking, to make sure that they don't dry out. We would suggest baking them for around 20 minutes, but start checking after about 17 minutes. They will be baked when they are set on top and a cake tester inserted into the centre comes out with a few damp crumbs clinging to it, but not uncooked batter. However please note that we have not tried the recipe in this form, so we are unable to guarantee the results.