Round Corners Customisation Script for Adobe Illustrator. What’s wrong with the standard rounding and much more about the script is explained in detail in the article. (Script Author)
The script does not round corners by itself, but modifies already rounded corners. It modifies all corners that are selected when you run the script.
This is not exactly squircle, but pretty close to it.
A squircle is a shape intermediate between a square and a circle. There are at least two definitions of “squircle” in use, the most common of which is based on the superellipse. The word “squircle” is a portmanteau of the words “square” and “circle”. Squircles have been applied in design and optics.(wiki)