Preheat and Prepare: Set your oven to 180°C (350°F), or 160°C (320°F) if using a fan-forced oven. Arrange oven racks in the upper and lower thirds. Lightly grease 3 baking sheets and line them with parchment paper.
Cook the Dough Base: In a medium saucepan, melt the cubed butter over medium heat. Once it’s almost melted, add the water and bring the mixture to a gentle simmer.
Add Flour and Form Dough: Remove the pan briefly from heat, then stir in the flour and salt all at once. Return the pan to medium heat and stir vigorously with a wooden spoon. The dough will become pasty and then quickly form a sticky ball that pulls away from the sides. Remove from heat and let it cool for exactly 10 minutes.
Incorporate Eggs: Add eggs one by one to the cooled dough, stirring vigorously after each addition until fully combined. The dough will appear separated at first but will come together into a thick, sticky paste that holds its shape when done.
Pipe the Pastry: Fill a piping bag fitted with a 15 mm star tip with the dough. Pipe long logs about 12-14 cm (5 inches) in length, 2.5 cm (1 inch) wide, spaced about 5 cm (2 inches) apart on the baking sheets to allow room for puffing.
Bake and Crisp: Bake two trays simultaneously for 45 minutes. At 30, 35, and 40 minutes, briefly open the oven door for 2 seconds to release steam — this helps create crisp pastry. Remove the trays after 45 minutes and allow the éclairs to cool completely on the trays.
Prepare Chocolate Glaze: Place chopped chocolate in a heatproof bowl. Warm the cream and corn syrup together in a small saucepan until just below boiling. Pour the hot cream mixture over the chocolate and let it sit for 1 minute. Stir gently with a rubber spatula until smooth and glossy.
Dip Éclair Tops: Carefully dip the top half of each cooled éclair into the chocolate glaze, letting excess drip off. Place glazed tops on a tray and refrigerate for 30 minutes until set.
Whip the Filling: Combine heavy cream, mascarpone, powdered sugar, and vanilla in a chilled bowl. Using an electric mixer on medium-high speed, whip until stiff peaks form, creating a thick, stable cream.
Assemble the Éclairs: Slice each éclair horizontally. Using a piping bag fitted with a star tip, pipe generous amounts of the stabilized cream into the base. Place the chocolate-coated top back on and serve immediately or chill until serving.