How to Make It
Stir together brown sugar, melted butter, and ¼ teaspoon of the salt in a small bowl. Generously coat a 6- to 7-quart round or oval slow cooker with cooking spray. Spread brown sugar mixture evenly in bottom of slow cooker. Arrange pineapple slices over brown sugar mixture, trimming as necessary to make an even layer.
Beat granulated sugar, oil, egg, and egg yolk in a medium bowl with an electric mixer on medium speed until slightly thickened and smooth, about 2 minutes. Stir in vanilla. Whisk together flour, baking powder, baking soda, and remaining ¼ teaspoon salt in a medium bowl. Stir half of the flour mixture into batter until almost incorporated. Add milk, and stir to combine. Add remaining flour mixture, and stir just until incorporated. Spoon batter over pineapples, and spread to edges of slow cooker with an offset spatula.
Drape a clean kitchen towel over slow cooker, and place lid over towel. Pull towel taut under lid. Cook on LOW until a wooden pick inserted in center comes out clean, about 2 hours and 30 minutes. Remove slow-cooker insert to a wire rack, and let stand until cool enough to handle, about 20 minutes. Place a cutting board or serving plate over slow-cooker insert, and carefully invert to release cake. Serve warm or at room temperature.