Preheat the oven to 220°C (200°C fan-forced). Line two baking trays with baking paper.
To make the choux pastry, sift flour and salt three times into a medium bowl.
Combine water, milk and butter in a medium saucepan over a medium heat. Once butter has melted, bring to the boil and add flour mixture quickly, stirring vigorously with a wooden spoon, until mixture comes away from the sides of the pan. Remove from heat and continue to beat for about 30 seconds or until mixture is smooth a. Set aside and allow to cool for 5 minutes.
Using a wooden spoon, gradually add beaten eggs a little at a time, beating vigorously until incorporated and the mixture becomes glossy and the consistency of a very thick cake batter.
Transfer mixture to a piping bag and pipe 8 x 10cm logs onto each prepared baking tray.
Place each tray one at a time into preheated oven and bake for 10 minutes, to allow eclairs to puff up. Reduce oven temperature to 200°C (180°C fan-forced) and bake for a further 15 minutes to dry out. Remove from oven and using a skewer poke a hole in the side of each eclair to allow the steam to escape. Using a serrated knife, halve each éclair and place back on baking tray with soft centres facing up. Bake for a further 5 minutes to help dry the centres out. Place on a wire rack and cool completely. Increase heat to 220°C (200°C fan-forced) and repeat process with remaining tray.
For the chocolate crème patissiere, combine the egg yolks, sugar and cornflour together in a medium mixing bowl and beat together using electric beaters until mousse like. Heat the milk in a small saucepan over a medium heat, to just below boiling point. Slowly pour the hot milk over the egg mixture, stirring continuously until well combined. Transfer the mixture back to the saucepan, using a spatula to scrape the mixing bowl. Place saucepan back over a medium heat, stirring egg mixture constantly until it thickens and becomes glossy. Remove from heat and allow to stand for 5 minutes before adding milk chocolate pastilles. Stir until melted and combined. Place in a small mixing bowl, covered in plastic wrap and allow to cool. Set aside until required.
Meanwhile for the glaze, combine sugar and 1 ½ tablespoons water in a small saucepan over low heat and stir until sugar dissolves. Increase heat to medium and cook for 5 minutes, oruntil a light caramel colour. Remove from heat, add remaining water and cream, to stop the mixture caramelizing. Add chocolate, return mixture to a low heat, stirring until smooth and combined, set aside and keep warm.
To prepare the eclairs, carefully dip each top in the chocolate glaze and place on a wire rack to set. Place milk chocolate crème patissiere in a piping bag and fill each bottom half of the eclairs. Sandwich together with the dark chocolate glazed tops. Serve immediately.