I have been making impossible pies, both savoury and sweet since I first came across them in the 70s. They appear to have had a resurgence of interest recently. The main difference is the egg whites are now separated, whipped separately and folded in to the well mixed yolk mix which creates a much softer custard layer.
This makes the recipe a more bit complicated and creates a lot of washing up but it takes the impossible pie to a new level. The custard actually separates, the cake layer is soft and fluffy and the base is a pastry like dense layer.
It works if the instructions are followed which includes having all the ingredients at room temperature.
Ensure the tray is small enough to get enough height for the custard layer to form. If necessary add some extra height with baking paper.
If making a savoury version you can double all the ingredients and use a well greased baking pan or casserole dish.
For the plain one I used a 20cm round pan but a long loaf pan or a square one that can be lined with a single piece of baking paper would have been better. The mixture is thin and it leaked under the paper. It was a bit tricky to take out the cake, peel off the paper and lower it back on to the custard layer.
Don’t use a spring form pan – the mixture is too runny and will leak.