For mocha crème pâtissière, bring milk and coffee to the boil in a saucepan over medium-high heat. Meanwhile, whisk yolks and sugar in a bowl to combine, then whisk in flour. Add milk mixture, whisking continuously until smooth, then return to pan. Whisk over medium heat until thick and smooth (3-4 minutes), add chocolate, remove from heat and whisk to combine. Transfer to a bowl, cover closely with plastic wrap and refrigerate until chilled, then fold through whipped cream and transfer to a piping bag (no nozzle). Refrigerate until required.
Preheat oven to 220C. Combine butter and 250ml water in a saucepan, bring to the simmer over high heat and cook until butter is melted. Add flour, beating vigorously with a wooden spoon until mixture pulls away from sides of pan. Remove from heat, cool for 5 minutes, then add eggs one at a time, beating well to combine after each addition. Transfer to a piping bag fitted with a 1.5cm plain nozzle and pipe 8cm lengths on trays lined with baking paper. Bake for 10 minutes, then reduce oven to 180C. Prick base of pastries with tip of a small knife and bake until dry and light golden (5-10 minutes). Transfer to a wire rack, cool completely then halve horizontally with a bread knife.
For dark chocolate glaze, combine sugar and 60ml water in a small saucepan, stir over medium-high heat until sugar dissolves, then cook until light caramel (8-10 minutes). Remove from heat, add chocolate and 50ml water (be careful, hot caramel may spit), return to heat and stir until smooth. Add another 50ml water, stir to combine, then strain through a fine metal sieve into a heatproof bowl. Keeping glaze warm over a saucepan of hot water, carefully dip éclair tops into glaze, shaking off excess, and set aside on a wire rack until set (10-15 minutes).
Pipe mocha crème pâtissière over éclair bases, sandwich with tops and serve.