In a medium saucepan, combine 60g of unsalted butter, 60g of water, 60g of whole milk, ¼ teaspoon of salt, and 1 teaspoon of sugar. Bring to a gentle simmer over medium heat. Remove from heat, then swiftly add 70g of sifted bread flour all at once while whisking vigorously until a cohesive ball forms. Return to heat and stir constantly for about 3-5 minutes until a thin film appears at the bottom of the pan.

Transfer the dough to a stand mixer and let it cool for a few minutes. Once slightly cooled, gradually add the pre-whisked eggs at low speed, mixing until the dough is shiny and pipeable. Aim for a smooth consistency—the dough should fall from the spatula in thick ribbons.

After preparing the dough, refrigerate it for 1-2 hours to firm up. Preheat your oven to 200°C (392°F). Use a piping bag fitted with a large round tip to pipe the dough onto a perforated baking mat or parchment-lined tray. Aim for 10cm long shapes, making sure to leave some space in between. Dust lightly with powdered sugar and freeze the eclairs for at least 1 hour.

Once the eclairs are well-chilled, lower the oven temperature to 170°C (338°F). Bake the eclairs for 40 minutes without opening the door for the first 30 minutes to prevent collapsing. They should be golden brown and puffed up. Allow them to cool completely on the baking mat.

In a bowl, whisk together 50g of sugar and egg yolks. Gradually stir in 12g of cornstarch, 12g of all-purpose flour, and 1 teaspoon of vanilla extract. In a saucepan, heat 240g of milk until just boiling, then temper the egg mixture by slowly adding the hot milk while whisking constantly. Return to low heat, stirring until it thickens into a creamy pastry filling, then mix in 15g of unsalted butter and let cool.

For the luscious chocolate glaze, melt 170g of semi-sweet chocolate in a heatproof bowl. In a small saucepan, heat 77g of heavy cream until hot but not boiling. Pour the cream over the melted chocolate and let sit for a minute. Stir until smooth and glossy, adding 15g of butter for extra shine.

Using a small knife, make a hole in each cooled eclair and pipe in the vanilla pastry cream until filled. Dip the tops of each filled eclair into the cooled ganache, allowing excess to drip off. Place them on a wire rack to set the glaze before serving.