My cake became densed at the bottom after I cooled it down. Can I know what have I done wrong to cause it like this? W a

There are so many reasons why this can happen. There is a comment about this under the recipe, and the author wrote: "The key to getting the signature airiness to this cake to whip the eggs as stiff as possible (just before they get overbeaten, it's a delicate balance for sure). " The whipped egg whites are the cake's only leavening. So if they're over-whipped, they'll collapse, and if they're under-whipped, there won't be enough structure to hold the cake up, and it will sink after it comes out of the oven. The author adds cream of tartar to the egg whites to stabilize the foam; I've found that a generous pinch of kosher salt will do the same thing. I hope this helps.