There are no shortages of button hover effects on here, this new design comes from Little Snippets. In this code snippet, you will see that the button has a thin line of border around the top right and bottom left corners. On hover, the border expands to fill the entire space around the button.
If you are having trouble with the pen, try the archived copy on GitHub
Expanding Border Button Snippet
See the Pen #1535 – Button by LittleSnippets.net (@littlesnippets) on CodePen.0