Cut the pineapple by removing the spiky top and slicing off the outer layer of skin. Cut the pineapple into spears, avoiding the center, or alternatively, core the pineapple and slice into rings. Lay them out on a baking sheet as you go.
Melt the butter and stir in the brown sugar and ½ tsp cinnamon and whisk until combined. Using a pastry brush, brush the pineapple spears with the glaze mixture. If any glaze is left, set aside.
Dust all sides slightly with additional cinnamon sugar if desired.
Add the glazed pineapples to a hot grill, laying them across the hot bars to get grill marks. Once charred on one side, flip over, roughly 3-5 minutes per side depending on the heat of your grill. Serve warm.