Nigella's Chocolate Guiness Cake is my entire family's favorite recipe. We have recently moved from Toronto to Denver. The altitude in Denver is over 5000 feet above sea level - an entire mile higher then Toronto. Everytime I've tried this favorite cake it puffs up, falls and turns out in a thin layer that looks life fudge. We really love this recipe and I would LOVE to have your suggestions for how to alter the recipe to be used in high altitude. My family and I both thank you for your help!
Baking at high altitude can be problematic but the main issue is that the lower air pressure at altitude causes the leavening in cakes to act too quickly, meaning they rise very fast but too quickly for the cake batter to set in the oven - so they expire quickly and the cake then sinks. Whilst we have not been able to test any cakes at high altitude there are a couple of adjustments that can be made to help prevent flat cakes.
First reduce the leavening by between 15 and 25% for altitudes above 5,000 feet/1500 metres (for the Chocolate Guinness Cake reduce the baking soda from 2 1/2 to to 2 teaspoons, if the cake is still coming out of the oven flat then try again with 1 3/4 teaspoons). Second increase the oven temperature by 25F/10c, the slightly hotter oven will help the cake batter to set more quickly and keep it aloft. But the cake will cook more quickly so make sure you start checking the cake 10 minutes before the end of the stated cooking time to see if it is done and then check regularly (for this cake you will need to increase the oven to 375F/190c and we suggest checking after 35 minutes).
Air can be dryer at altitude and sometimes extra moisture is needed. The Chocolate Guinness Cake has quite a lot of liquid in it so we don't think that you will need to add any extra in this cake but if cakes are coming out slightly dry then add 2 tablespoons extra liquid (such as milk) for each 150g/1 cup flour.