Preheat oven to 400°. On a rimmed baking sheet, turn pineapple in butter and brown sugar until coated. Spread in a single layer. Roast, turning as needed, until fruit has softened and is starting to brown, about 10 minutes. Put cake on another baking sheet and toast in oven until warm, about 5 minutes.
Combine cream, granulated sugar, and cinnamon in a medium mixing bowl and whip with a whisk until soft peaks form, about 2 minutes.
Put 1 cake slice on each dessert plate. Top cake with pineapple, using half. Drizzle with roasting juices and dollop with whipped cream. Repeat layers and top with a sprinkle of mint.