Preheat oven to 375°F. Line a baking sheet with parchment paper. Set aside.
On a lightly floured work surface, open puff pastry sheet and lay lengthwise. Spread mascarpone down center of puff pastry, leaving about 1 inch space on the top and bottom.
Whisk together sugar and cinnamon. Sprinkle half the cinnamon sugar mixture over mascarpone. Arrange peach slices evenly over mascarpone. Fold up top and bottom inch of puff pastry then fold left side over peaches then the right side of the puffed pastry to snugly secure all the filling.
Place on parchment lined baking sheet, seam side down. Brush dough with melted butter. Cut slits in top of dough about every 2 inches. Sprinkle with remaining cinnamon sugar.
Bake in preheated oven for 15-20 minutes, until puffed and golden brown. Remove from oven and serve warm.