Question: What system was developed by Margaret Weis Productions was used in a game based on the Supernatural TV series?

Context paragraphs:
Margaret Weis Productions: Margaret Weis Productions, Ltd. is a games publisher located in Williams Bay, Wisconsin, United States and founded in 2004 after Margaret Weis and Don Perrin, the two founders of Sovereign Press, Inc divorced.

Things Don't Go Smooth: Things Don't Go Smooth is the third title published for Margaret Weis Productions, Ltd's "Firefly Role-Playing Game".  It utilizes the Cortex Plus system.

Serenity Role Playing Game: Serenity Role Playing Game is a science fiction role-playing game released in 2005 and set in the universe of the movie "Serenity".  It was produced by Margaret Weis Productions, Ltd, and its mechanics were the first iteration of the Cortex System.  It won an Origins Award for best RPG in 2005 and Margaret Weis' license came to an end on January 31, 2011.

Marvel Heroic Roleplaying: Marvel Heroic Roleplaying (MHRP), is the fourth role playing game set in the Marvel Universe, published Margaret Weis Productions under license from Marvel Comics.  It was a fast playing game using the Cortex Plus system, with the first volume published in early 2012.  In early 2013 Margaret Weis Productions announced they would not be renewing their license.

Supernatural Role Playing Game: The Supernatural Role Playing Game is a role-playing game by Margaret Weis Productions that was released in 2009.  It is based on the television series "Supernatural" and was the final game to use the Cortex System.

Cortex Plus: The Cortex Plus System is a toolkit RPG system that evolved from Margaret Weis Productions, Ltd's Cortex System.  It has been used for four published games and one published preview to date, and the design principles are in the Cortex Plus Hacker's Guide, a book of advice in how to create new games using Cortex Plus, and list of new games produced via kickstarter.  According to the Hacker's Guide there are three basic 'flavors' of Cortex Plus; Action, Drama, and Heroic.

Dragonlance: Dragonlance is a shared universe created by Laura and Tracy Hickman, and expanded by Tracy Hickman and Margaret Weis under the direction of TSR, Inc. into a series of fantasy novels.  The Hickmans conceived "Dragonlance" while driving in their car on the way to TSR for a job interview.  At TSR Tracy Hickman met Margaret Weis, his future writing partner, and they gathered a group of associates to play the "Dungeons & Dragons" role-playing game.  The adventures during that game inspired a series of gaming modules, a series of novels, licensed products such as board games, and lead miniature figures.

Firefly Role-Playing Game: The Firefly Role-Playing Game is a science fiction role-playing game released in 2014 and set in the universe of the Joss Whedon television show "Firefly".  It was produced by Margaret Weis Productions, Ltd, and uses the "Cortex Action" variant of Margaret Weis Production's proprietary Cortex Plus game system.

Smuggler's Guide to the Rim: Smuggler's Guide to the Rim is the fourth title published for Margaret Weis Productions, Ltd's "Firefly Role-Playing Game".  It presents several planetary systems in the "Firefly" game setting in gazetteer format and expands the game's rules.

Cortex System: The Cortex System is a generic RPG system based on the "Sovereign Stone System ", and was developed by Margaret Weis Productions, Ltd for the Serenity Role Playing Game.  It was subsequently used for their licensed "Battlestar Galactica" and Supernatural RPGs, and brought out as a stand-alone system in the "Cortex System Role Playing Game" book (also called the "Cortex Classic System Role Playing Game").  Serenity, using the Cortex System, was the 2005 Origins Award Gamer's Choice Role Playing Game of the Year.

Supporting paragraphs: Cortex System Supernatural Role Playing Game
Answer: The Cortex System