I recently baked a lemon mascarpone cake (recipe from lovelifeandsugar). I tweaked the recipe a bit by using yoghurt instead of sour cream (because I didn't have any) and used two tablespoons of oil instead of sugar. I also whipped my egg whites separately before folding them in. The cake turned out really tasty and moist except... when I tried to frost it it crumbled away. I'm wondering how do people get such study cakes like what you see in videos and photos? I tried to mix as little as possible after I added the flour ( I did it by hand after I added the dry portion in). I did put it in the fridge, but it didn't help much. Any suggestions will be greatly appreciated.

I always bake my cakes a day before I ice them.  I wrap while still warm in saran wrap, let them sit overnight and then level, fill & ice them the next day.  It always helps with crumbling. Not sure if this method will help... because I never made this cake before.  

why did you mix it little after  adding the flour -- you have to incorporate the ingredients well at some point -- after adding in the beaten egg whites you don't want to mix just fold though -- 

I don't understand these two things:

I tried to mix as little as possible after I added the flour

and used two tablespoons of oil instead of sugar