In a medium saucepan, add the granulated sugar, cornstarch, and salt; whisk well to combine. All at once, add in the egg yolks and whisk until smooth. Add in 1/3 cup of the milk and whisk until combined; whisk in the remaining milk. Place the pan over medium heat. Cook the mixture, stirring constantly with a whisk, until thick and creamy, about 8 to 10 minutes. You will know the mixture is done when it's thick enough to coat the back of a spoon and almost resembles pudding. Remove pan from heat and whisk in the butter and vanilla; beat until smooth. Transfer the custard to a heatproof bowl and cover tightly with plastic wrap. Be sure to press the plastic wrap right against the surface of the pudding to prevent "skin" from forming. Use a small knife to poke a few holes in the plastic. Place bowl in the refrigerator for 1 hour, or until completely chilled and firm.
For the Cream Puff Pastry:
Preheat oven to 400°(F). Line a large baking sheet with parchment paper; set aside.
In a large saucepan, combine the water and cubed butter; bring to a light boil. All at once, add the flour and salt, stirring constantly until a smooth ball forms. Remove from heat and allow the mixture to rest for 5 minutes (set a timer as this recipe is time sensitive). Add in the eggs, one at a time, beating well after each addition. Continue beating until mixture is completely smooth.
Drop rounded tablespoonfuls of the dough onto the prepared baking sheet, leaving 2 inches in between each for spreading. Bake for 30-35 minutes or until puffy and lightly golden brown. Transfer the warm puffs onto a cooling rack. Using a serrated knife, immediately - and carefully - slice the puff shells in half, horizontally. Gently gut and discard the soft dough from the bottom half of the shell (leave the tops of the shells as is). Set shells aside (tops and bottoms) to cool.
For the Chocolate Glaze:
Place the chopped chocolate in a small, heatproof bowl; set aside. In a small saucepan, heat the cream to a light simmer; pour warmed cream over the chopped chip. Allow the mixture to stand for 1 minute, then gently stir until the chocolate is fully melted and the mixture is completely smooth.
Assembly:
Fill each cream puff with a few tablespoons of the Boston cream; replace the tops. Drizzle a tablespoon of the chocolate glaze on top of each. Serve at once, or keep cream puffs stored in the fridge for up to 3 days.
Notes
*For best results, follow recipe exactly as written. *Cream puffs are best served the day they are made, but will keep, stored in the fridge for 3 days. *This recipe does not double. If you'd like more than one batch, make each one separately. *Do not over stuff the cream puffs or they will be very messy to eat. I recommend 2 to 3 heaping tablespoons of filling per puff.
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