People who bake cakes at home face the problem of a cake collapsing after it is taken out of the oven. There are a number of reasons for the cake to fall in the middle.
The oven temperature might be lower than needed for the cake to bake. Your oven can show a certain degree but inside the cavity the temperature can be lower. Get the oven checked.
Before removing the cake from the oven always insert a skewer just off center in the cake. If it comes out with batter clinging to it, it needs to be baked for some more time.
Always follow the recipe and put the exact amount of butter and sugar. No point in saving calories here as the proper rising of the cake depends on fat and sugar content.
It is also possible that your baking power is old or no longer effective. You can test its effectiveness by dissolving one teaspoon in a cup of hot water. If lots of tiny bubbles form and rise, it is still working. If not, replace it.