Add the ice cream, chocolate syrup, milk and peanut butter to a blender, cover, and quickly pulse on low to combine. Continue pulsing until you see a whirlpool form—that's when you know it's done. Pour or spoon the shake into two tall glasses. Top with any garnish you'd like—cherries, whipped cream, shaved chocolate, Lipitor, whatever you damned well please. It's your party. Own it! Serve with straws and spoons.
Chocolate Ice Cream:
Warm the milk and cream in a medium saucepan over medium heat, stirring frequently, until the mixture begins to steam. Add the sugar and stir until completely dissolved.
Meanwhile, warm the chocolate in a microwave for about 30 seconds to melt it. Add the chocolate to the saucepan and stir for about 3 minutes, until thoroughly combined. Whisk in the cocoa powder and salt until dissolved. Then reduce the heat and cook at a low temperature, stirring, for 2 to 3 minutes. Remove from the heat and stir in the Grand Marnier, if using.
Transfer the mixture to a 1-quart container, cover, and refrigerate for at least 6 hours. The chilled mixture will be quite thick, so give it a good whisk before pouring it into your ice cream maker. Freeze according to the manufacturer’s instructions. When it is finished, transfer the ice cream to an airtight container and freeze for at least 4 hours to harden. Makes 1 quart