Preheat oven to 350 F. Line muffin tin with 12 paper muffin cups. Sift together the flour, salt and baking powder. In another bowl, add the sugar, melted butter, peanut butter and eggs and mix until well combined. Stir in the dry ingredients and the milk until evenly moistened. Do not overmix. The batter will be slightly lumpy. Fold in the grapes. Spoon the batter into the prepared muffins cups and top with a teaspoon of grape jelly. Bake in the preheated oven until the muffins are golden brown, 14-16 minutes. Cool before serving.