Why does the added fruits and / or nuts ... sink with a cake, what can I do about this?
The reason for this is usually the butter or margarine is beaten too light. Then the rest of the raw materials are added and the batter is again beaten too light. The volume of the cake batter is increased, but not the weight. The result is that with a light batter the added fruits / nuts are heavier than the batter. The cake batter bakes up through the fruits and the fruits / nuts remain at the bottom.
The solution is to simply stir the butter until its creamy or, better yet, let it melt gently. Stir in the rest of the raw materials (except the fruits / nuts). Let the batter rest for 10 minutes and the fruits / nuts are mixed underneath. The desired result is obtained by applying this method.
Alternatively you can try and coats the nuts and fruit in a small amount of flour and this will help it stick to the batter.