Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a medium saucepan, combine 1 stick butter, 1 cup water, and ¼ tsp salt. Bring to a boil over medium heat.

Once boiling, reduce the heat to low and add 1 cup all-purpose flour all at once. Stir vigorously with a wooden spoon until the dough pulls away from the sides and forms a smooth ball (about 1–2 minutes).

Remove from heat and let the mixture cool for 5 minutes.

Beat in 4 eggs, one at a time, fully incorporating each before adding the next. The dough should be smooth, glossy, and pipeable.

Transfer the dough to a piping bag fitted with a large round or French tip. Pipe 4–5 inch long logs onto the prepared baking sheet, leaving space between each.

Bake for 25–30 minutes, or until golden brown and puffed. Do not open the oven during baking.

Remove and let cool completely on a wire rack.

In a medium saucepan over medium heat, warm 2 cups whole milk and ¼ cup semi-sweet chocolate chips until just steaming—do not boil. Stir to melt the chocolate.

In a separate bowl, whisk together ½ cup sugar, ¼ cup cornstarch, ¼ tsp salt, and 4 egg yolks until smooth and pale.

Slowly pour the hot milk mixture into the egg mixture, whisking constantly to temper the eggs.

Return the mixture to the saucepan and cook over medium heat, whisking constantly, until it thickens into a pudding-like consistency (about 5 minutes).

Remove from heat and stir in 2 Tbsp butter and 2 tsp hazelnut extract.

Transfer pastry cream to a bowl, cover with plastic wrap (press it directly onto the surface), and chill for at least 30 minutes.

In a small saucepan or microwave, heat ½ cup heavy cream until just steaming.

Pour over ½ cup semi-sweet chocolate chips in a heatproof bowl. Let sit for 1–2 minutes, then stir until smooth and glossy. Set aside to cool slightly.

Once the choux shells are fully cooled, use a small knife to cut them in half lengthwise or poke holes in the bottom for filling.

Fill a piping bag with the chilled hazelnut pastry cream and pipe it generously into each éclair.

Dip the tops of the éclairs into the ganache or spoon it over the top for a glossy finish.

Chill for 30–60 minutes to set, then serve and enjoy!