Whole milk (full-fat milk) is recommended as it provides a creamier texture. But you can substitute with any milk of your choice.
Dutch-processed cocoa powder is recommended as it offers better flavour & quality.
I usually use baking dark chocolate but you can use any commercial chocolate bar as long as it's dark chocolate. The amount to be added is flexible as different brands of chocolate could give differ in richness & flavour.
Do not bring the hot chocolate mixture to a boil as the chocolate flavours could fade away as a result of the high temperature.