Spray the inside of the slow cooker crock with cooking spray.
In a large bowl, stir together flour, baking powder, salt, and 3/4 cup white sugar.
Stir milk, melted butter and vanilla into the flour mixture until smooth. Pour into the greased crock.
In a small bowl, combine brown sugar and kosher salt with a fork. Sprinkle over the top of the batter in the crock.
Pour hot water over the top of ingredients in the slow cooker. Do not stir!
Put the lid on the slow cooker and cook on low for 3-4 hours. Remove the lid and let stand for several minutes before serving. At first, it will be a bit too liquidy, but should thicken up after several minutes.