Preheat oven to 375 degrees F (190 degrees C). Line a muffin pan with paper muffin liners, and lightly spray liners with cooking spray.
Melt butter in a heavy saucepan over medium heat until golden brown. Remove from heat and stir in pineapple. Set aside to steep for 20 minutes.
Whisk flour, cornmeal, baking soda, and salt until combined. Set aside.
Combine buttermilk, eggs, and pineapple mixture in a large bowl. Whisk until smooth, about 1 minute. Add flour mixture to buttermilk mixture; whisk until just combined.
Divide batter evenly between the prepared muffin cups and bake in the preheated oven until a toothpick inserted in the center of a muffin comes out clean, about 25 minutes. Cool in the pan for 5 minutes before removing to cool completely on a wire rack.