Preheat a gas grill to medium heat (400-450F). Clean the grates.
Brush pineapple slices with melted butter. Place pineapple on the grill and grill for 4 minutes on one side, until you get golden brown grill marks and the pineapple can be easily flipped around. Grill for about 4 minutes on the other side. Transfer to a plate.
For the mascarpone whipped cream - beat mascarpone and powdered sweetener with an electric mixer for 2 minutes. Slowly add the heavy cream, beating on low speed. Increase the speed and beat for 30-60 seconds, until thick. Beat in the lemon zest. Add cinnamon (optional)
Serve pineapple slices with a scoop of mascarpone whipped cream. Dust the top with cinnamon (optional).