1. In a medium bowl, combine sugar, cornstarch, cocoa powder and salt. Whisk in 1/2 cup milk and egg yolks until blended and smooth.
2. Pour remaining milk into a large saucepan and bring to a simmer over medium-high heat. Whisking vigorously and constantly, pour about 2 cups hot milk into sugar mixture until blended. Pour mixture back into saucepan with remaining milk, whisking well.
3. Reduce heat to medium and cook, stirring constantly, until smooth and thickened, about 2 minutes. Remove from heat and whisk in vanilla and 1/2 of chocolate. Stir until chocolate has melted. Divide pudding among 6 1-cup ramekins or custard cups. Place remaining chocolate in a microwave-safe bowl and heat on high for about 30 seconds, stirring every 10 seconds, until melted.
4. Place graham crackers in a ziplock bag; seal bag. Use a rolling pin to crush into crumbs. Sprinkle top of each pudding with graham crackers and 5 miniature marshmallows. Drizzle with melted chocolate and serve.