How to make a square button with rounded corners.

I have found a lot of answers to the same question but it does not work. since the earliest answer I found was from 2015 I wonder if something has changed. using version 5.5.0f3

I have a picture of a square with rounded corners and have made it into a sprite (2D and UI). I pressed sprite editor and pulled in the green borders to the perfect place. I added the picture on a UI picture in the scene and set the image type to sliced.

after those steps I have a round button (the 4 corners of the picture are visible but nothing else) and when I scale in the rect transform the picture becomes an oval instead of a square with rounded corners. the standard UISprite Texture works fine and that's the effect I want with my own texture.

for now I might just let it as it is but it looks like the function is suppose to exist.