• Step 1: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
• Step 2: Unfold the puff pastry sheet and cut it into 12 equal rectangles. Place the rectangles on the prepared baking sheet, spacing them apart. Bake for 15-18 minutes, until golden brown. Allow the eclairs to cool completely on a wire rack.
• Step 3: In a medium saucepan, combine the milk, sugar, and cornstarch. Whisk together over medium heat until the mixture thickens and comes to a boil, about 5-7 minutes. Remove from heat.
• Step 4: In a separate bowl, whisk the egg yolks. Slowly pour in 1/2 cup of the hot milk mixture, whisking constantly. Then pour the egg mixture back into the saucepan with the remaining milk mixture. Cook over medium heat, stirring constantly, until the custard thickens, about 2-3 minutes.
• Step 5: Remove the custard from the heat and stir in the butter and vanilla extract. Pour the custard into a clean bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and refrigerate for at least 2 hours, or until completely chilled and thickened.
• Step 6: In a small saucepan, heat the heavy cream over medium heat until just simmering. Remove from heat and stir in the chocolate chips until melted and smooth. Allow the chocolate glaze to cool slightly, about 5 minutes.
• Step 7: Using a serrated knife, slice the cooled eclairs in half horizontally. Spoon or pipe the chilled custard onto the bottom halves of the eclairs. Top with the top halves, then drizzle the chocolate glaze over the tops, allowing it to drip down the sides.
• Step 8: Refrigerate the assembled eclairs for at least 30 minutes before serving to allow the glaze to set.