My cake Coffee And Walnut Cake did not rise evenly and when I let it cool, it slumps down. I wonder if substituting bicarbonate soda with 2 tsp of baking powder caused this? I could not find bicarbonate soda at the supermarkets near where I live. Also I did not have 2 sandwich tins at hand so I just put the mixture into a single silicone cake mould and I baked it for 25 minutes - is this alright? What would you recommend I do? I'm just a beginner in baking. Thank you!
Nigella's Coffee And Walnut Layer Cake (from Kitchen and on the Nigella website) uses a combination of baking powder and bicarbonate of soda (baking soda). The bicarbonate of soda gives the cake slightly extra "lift" to offset the ground nuts in the mixture (which could give a slightly denser texture). If you have substituted the 1/2 teaspoon bicarbonate of soda with 2 teaspoons of baking powder then there will be far too much raising agent and the cake will rise up a lot in baking but then collapse on cooling.
If you have also used one large cake mould rather than two sandwich tins (cake pans) then the baking time is also likely to be too short at 25 minutes, so the other possible cause of the cake sinking is that it was not cooked properly in the centre. If you are new to baking and would like to do more then it is worth buying a pair of 20cm (8-inch) round tins as these are used in many cake recipes. Loose-bottomed tins are popular in the UK while solid-bottomed tins are more poular in the US. Either type is good for this recipe and for most layer cake recipes. You can get this type of tin in many supermarkets, as well as cake decorating stores.