Each time I bake Buttermilk Birthday cake (in Domestic Goddess) it separates. The cake bottom is fine, like a flour cake should be, but the top is a rubbery eggy consistency, like an egg tart with no flour! How do you stop this from happening and achieve a perfect unanimous cake texture? My thought was to add small amount of vinegar to react with the bicarbonate of soda - do you think this would work?
Nigella's Buttermilk Birthday Cake (from HOW TO BE A DOMESTIC GODDESS) is a vanilla cake that does not have any particularly unusual ingredients and the method is fairly standard. It should produce a firm but moist and slightly fluffy cake. It has been used many times for sandwich cakes and also for shaped birthday cakes.
As the buttermillk is acidic this is enough to react with the bicarbonate of soda (baking soda) and no extra acid should be needed. Also the main raising agent is baking powder, which does not need acid, and the bicarbonate of soda is there just to give a slight extra boost. If you were using regular milk instead of buttermilk then you would need to add a little lemon juice or a mild vinegar, but buttermilk, or a yogurt-milk mixture, should be acidic enough. We wonder if the oven is set correctly as it should be 180c/160 Fan/350F. If the oven is too cool then the batter can separate during baking and the flour mixture will sink to the bottom as it is heaviest. The eggs and liquid are less dense and will sit on top, setting into a custard layer. We would suggest checking the oven to make sure that it is running at the correct temperature, is fully preheated and bake the cake on the middle shelf. Also make sure that the cake batter is thoroughly (but not overly) mixed. If using a stand mixer you may want to give the cake batter a gentle couple of turns with a spatula before the batter is transferred to the cake pan. Sometimes the paddle or whisk attachment on the mixer does not quite fully mix in the ingredients at the very bottom of the bowl and this could also be a factor.