For vanilla-caramel crème pâtissière, stir 150gm sugar and 80ml water in a saucepan over medium-high heat until sugar dissolves, bring to the boil and cook, without stirring, until dark caramel (3-4 minutes). Remove from heat, add milk (be careful as hot caramel will spit), then vanilla bean and seeds, return to heat and whisk to combine. Meanwhile, whisk yolks and remaining sugar in a bowl until pale (2-3 minutes), add cornflour and whisk to combine. Strain milk mixture over egg mixture, whisking continuously, then return to pan and whisk continuously over medium heat until thick and smooth (2-3 minutes). Transfer to a bowl, cover closely with plastic wrap and refrigerate to chill. Whisk until smooth, fold in whipped cream and refrigerate until required.
For choux pastry, preheat oven to 220C. Bring butter and 185ml water to the boil in a saucepan over medium-high heat, ensuring all butter melts. Add flour, beat continuously until mixture is thick and smooth and pulls away from sides of pan (1 minute), remove from heat and stand to cool (10 minutes). Add eggs one at a time, beating well after each addition. Spoon into a piping bag fitted with a 12mm plain nozzle and pipe into 10cm lengths on an oven tray lined with baking paper, leaving space between each for pastry to puff. Bake for 20 minutes, reduce heat to 180C, prick bases with a skewer and bake until golden and dry (15-20 minutes). Cool to room temperature on a wire rack.
Meanwhile, combine sugar and 60ml water in a small saucepan over medium-high heat and stir until sugar dissolves, then cook, without stirring, until light caramel (6-8 minutes). Remove from heat, add cream and 60ml water (be careful as hot caramel will spit), add chocolate, return to heat, stir until smooth and combined, set aside and keep warm.
Halve éclairs horizontally with a bread knife, carefully dip tops in chocolate glaze and place on a tray to set. Transfer vanilla-caramel crème pâtissière to a piping bag fitted with a 1cm plain nozzle, pipe over éclair bases, sandwich with tops and serve.