For Chocolate Dairy Base: Blend the granulated sugar in a bowl with the powdered milk and sifted cocoa powder. Warm half of the milk to just below the boil, whisk in dry ingredients and stir to dissolve the sugar. Add the remaining ingredients for the base then chill. Refrigerate up to 3 days or until ready to use.
For Beverage: Combine first 5 ingredients in blender container; cover. Blend until smooth.
Pour into chilled tall glass. Garnish with whipped cream and additional OREO Pieces.