Preheat the oven to 350°F (175°C) and line a cupcake pan with paper liners.
Melt the butter in a saucepan over medium heat; stir in brown sugar until dissolved.
Pour the mixture evenly into the bottom of the cupcake liners.
Place a pineapple slice on top of the butter mixture in each liner.
In a mixing bowl, combine the cake mix, eggs, crushed pineapple, milk, and vanilla extract.
Mix until all ingredients are well combined and smooth.
Pour the cake batter over the pineapple slices in each liner.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Let the cupcakes cool in the pan for 10 minutes.
Invert onto a serving platter and let cool completely before serving.