I just brewed the chocolate hazelnut porter from the Brewing Classic Styles book last month. The chocolate flavor in that recipe comes from unsweetened cocoa powder. Haven't bottled mine yet, but the chocolate aroma and flavor was evident in the hydro samples I've taken.
Of course, the cocoa powder is "unsweetened". Using lactose in addition to the cocoa might lend the sweet chocolate flavor you're looking for. I've used lactose and cocoa powder in beers, just not in the same beer, so can't say for sure.