Why do the bottoms of my cookies look like cakes? What could possibly have gone wrong? I used only 1 egg yolk with no baking soda and Melted butter:

 

And this is what I got after beating sugar+melted butter+yolk:

   

After adding dry ingredients:

 

And here's what it looks like in the oven:



In which step do you think I could have done something wrong? I have no idea really.
These cookies look all right, there is no need to change anything in your technique to correct the looks. 

If there is a problem with the texture, then this is a different question (which, as far as I can remember, we already helped you with in another thread). But the only viable answer to this question as stated is: no need to change anything here.