For the chocolate decorations: Put the melted chocolate into a pastry bag and snip off a tiny hole at the tip. Pipe lacy chocolate designs--just larger than the tea cups you're using--onto a baking mat or piece of parchment paper. Allow to set completely.
For the hot chocolate: Combine the milk and half-and-half in a small saucepan. Warm over medium-low heat, then stir in the bittersweet and semisweet chocolate chips, sugar and vanilla, heating until melted and combined. Transfer the mixture to a teapot.
To serve: place a lacy chocolate piece over each tea cup and pour over the hot chocolate. When the chocolate has melted into the cup, top with the whipped cream and enjoy.