In a medium saucepan, gently warm half & half and sugar, stirring often at first, then frequently, til small bubbles form along the edge.
Meanwhile, in a separate bowl, whisk yolks til smooth, then whisk in malted milk powder and cocoa powder until fully incorporated. A quarter cup at a time, add hot milk mixture to egg mixture, whisking well after each addition. Return mixture to pan and cook over medium heat, stirring nearly constantly, until it begins to thicken. (Experienced custard makers may stir less often though at risk of scorching the mixture if time gets away.) Remove from heat and stir in vanilla. Transfer to a refrigerator container, let cool, cover and refrigerate for 8 - 24 hours.
Process the cold mixture in a commercial ice cream processor until thick. Serve immediately or hold in the freezer for an hour.
For chocolate malts, mix chocolate malt ice cream with milk in a blender.
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