The stitch used is called the single crochet thermal stitch; I’ve used it before. It’s also commonly called the “potholder stitch”.

Good video tutorial: Single Crochet Thermal Stitch Tutorial

This is an easy stitch to learn but the first 3 rows are hardest, as far as being able to find the beginning and end sts. Put a stitch marker them, and it will save headaches. After row 5 it’s much better.

The stitch is slow going, especially with the smaller size G hook I used (I also crochet tighter), but it makes a very dense fabric; no way will heat penetrate.

Worsted cotton is not as slide-y as acrylic, but it is what you want for a potholder or hot pad (acryic can melt). But the end result is WORTH IT. This is a very nice, thick, well-shaped, sturdy potholder that will last a LONG time and I would be proud to give as a gift.