While the grill is heating, combine the frozen berries, sugar, lemon juice and 1/4cup of water in a small saucepan. Cook until the mixture starts to simmer. Mash the berries with the back of a spoon.
Whisk cornstarch with 1tbsp water; add it to the berry mixture. Cook for 1 minute or until the mixture thickens. Turn off the heat and set aside.
Brush the pound cake with the melted butter and place the slices on the hot grill. Make sure to turn the slice a little to get the nice grill marks. Flip and cook on the other side. This takes about 1~2 minutes per side.
Serve the grilled cake with the berry sauce and more fresh berries on the side. Enjoy!!