Slice pineapple into eight rings and cut each into 6 pieces. (If you can't find cored pineapple, buy a whole one and use a paring knife to remove skin and cut into slices. Extract core from each slice with a round cookie cutter.)
Warm butter in a large skillet over medium-high heat. When butter begins to foam, add pineapple and brown sugar and cook, stirring frequently, until pineapple begins to brown and sugar is dissolved, 2 to 4 minutes. Remove from heat, scrape into a shallow bowl, and let stand until just warm to the touch, 2 to 3 minutes.
Divide ice cream among 6 dessert dishes. Top each with pineapple and sauce. Serve immediately.