Step 1In a medium saucepan, combine 1 cup water with the sugar and corn syrup. Bring the mixture to a boil, stirring occasionally. Turn off the heat.
Step 2Stir in the cocoa powder and chopped chocolate. Remove from heat and allow the chocolate to melt completely, whisking occasionally as it cools.
Step 3Strain the sauce and cool. (Whisk again before using.)
Step 1Make the shakes in two batches. In a blender, combine four (4-ounce) rounded scoops of ice cream, 2 tablespoons chocolate sauce and one-half cup milk. (For malts, add 2 tablespoons malted milk powder.) Blend until smooth. Pour into two glasses. Repeat.