The best hot chocolate, I found was at Ivy Bros, served in a white mug with bumpy texture on outside and smooth blue inside. Promisingly it was described on the menu as melting hot chocolate. Indeed it was good pure chocolately drink. The cafe itself is quirky with electric objects which they sell and a shark jaw above the counter. They offer an interesting and tempting range of sandwiches and cakes.
It is at Oudezijds Vootmrburgusal 96HE 1012 GH