The museum quarter is short on great chocolate, but not as short as the guidebooks make out. From the Rijksmuseum's front steps you're within a 10-minute walk of three shops that are worth the detour and one tourist trap that isn't.
Head north over the museum bridge, follow Spiegelgracht, turn left onto Leidsestraat. Number 87 on the right. Handmade truffles and single-origin bars in a hidden atelier. Open daily 10:00–22:00.
Longer walk but worth it if you want the classic Amsterdam bonbon experience. Cross the canal belt via Vijzelstraat and continue east.
Old-school Belgian-style pralines in a 1970s tearoom. Sit down with a hot chocolate after the museum.
The chocolate window directly opposite the Rijksmuseum entrance. Tourist prices, industrial chocolate. The walk to any of the three above is short enough that there's no reason to settle.
Amsterdam Chocolate Factory on Leidsestraat 87 is the closest handmade chocolatier, about a 7-minute walk north of the museum.
No, but the museum café sells a small selection of Dutch bars. For a real chocolate stop, walk 7 minutes to Leidsestraat.