1. In a large saucepan, heat the pareve milk over low heat until bubbles begin to form at the edge. Do not bring to a boil or it will curdle. Stir in ¼ cup sugar until dissolved.
2. In a small bowl, whisk the egg yolks and remaining ¼ cup sugar. Mix in the melted chocolate. Whisk in a small amount of the heated milk, then transfer the chocolate mixture to the saucepan. Cook the mixture over low heat, whisking frequently for 15 minutes. At this point the mixture will be thick enough to coat the back of a metal spoon.
3. Remove from heat and cool quickly by placing the saucepan in a large bowl filled with ice water. Allow to stand for 30 minutes, stirring mixture periodically.
4. Transfer mixture to a large bowl and stir in pareve whipping cream and vanilla extract.
5. Divide chocolate gelato between 12 prepared glasses or bowls. Freeze until firm.
6. Place the chocolate sandwich cookies in a plastic bag and hit with a rolling pin to break into chunks. Sprinkle over chocolate custard. Add marshmallows and top with the chopped Viennese crunch and almonds, if desired. Drizzle with chocolate syrup. Freeze until ready to serve.