Line slow cooker with aluminum foil to create a foil bowl.
In a large mixing bowl, stir together melted butter and brown sugar until smooth. Add vanilla and eggs and mix until smooth. Add flour and salt, stirring until well combined.
Pour batter into the bottom of the slow cooker. Sprinkle chocolate chips on top. Secure a paper towel under the lid of the slow cooker.
Cover and cook on high 2½ - 3 hours or until set in the middle. Start checking at the 2 hour mark, and rotate crock if it looks like the sides are getting too brown.
Cool at least 1 hour before cutting to serve.Note: When testing ours was done in 2 hours 45 minutes.