The Knotted Buttonhole Stitch is a fancy variation of the regular Buttonhole Stitch or Blanket Stitch.

Start the stitch as with a regular Buttonhole Stitch, but wrapping the thread around the needle (Step 1). You can make the wrap with your fingers, or by twisting the needle around. Next, insert the needle with the thread under the needle (Step 2). Pull the thread taut before you draw the needle through.