If you notice in step three of the directions, I said to whip until stiff peaks form, but I prefer to whip it a little more than stiff peaks because once you add in the sweetened condensed milk, the peaks will deflate a little bit. This way you can get nice stiff peaks towards the end. Another option is to fold in the condensed milk without using the mixer, but using a mixer is always easier.
I use store-bought chocolate chip cookies and Oreos. You can use any type of cookies you want. I don’t recommend using homemade cookies because the soft texture of the homemade cookie won’t hold up well in ice cream.