Preheat a panini press or a grill pan. Sandwich the flatbreads together in 2 pairs. Brush the outsides of the sandwiches with the butter and sprinkle generously with the sugar. Toast in the paini press over high heat until golden and caramelized, about 4 minutes. Alternatively, grill the sandwiches, pressing with a heavy skillet and turning once halfway through. Open the sandwiches and fill with the cream cheese, chocolate chips and pistachios. Close the sandwiches and let the chocolate melt. Cut the sandwiches into wedges and serve.