Coat each cup of a minibundtlette pan with cooking spray. Place 1 tablespoon brown sugar and 1 pineapple slice in the bottom of each cup; set aside.
Combine flour, baking powder, baking soda, and salt in a bowl; make a well in center of mixture. Combine buttermilk, syrup, oil, vanilla, and lemon juice; stir well. Add to flour mixture, stirring just until moistened.
Divide cake batter evenly among prepared minibundtlette cups. Bake at 350° for 30 minutes or until a wooden pick inserted in center of cakes comes out clean. Invert cakes onto a wire rack. Serve warm or at room temperature.