To make the choux pastry, preheat oven to 220 celsius (420 Fahrenheit) and line 2 trays with baking paper.
Place the butter and the water in a saucepan over medium-high heat. When the butter has melted add the flour and beat continuously until mixture is thick and smooth and pulls away from sides of pan in a ball ( roughly 1 minute). Remove from heat and place in the bowl of a stand mixer and allow to cool for 5 to 10 minutes.
Fit the mixer with a paddle attachment and beat. The mixture will look like it’s falling apart, but don’t fret, it will come back together when you add the eggs. Add the eggs, one at a time, and beat for 2 minutes, or until fully incorporated before adding the next egg. At the end you should have a relatively thick, but pipe-able dough. If the mixture is runny or sloppy keep beating.
Spoon the choux into a piping bag fitted with a 12mm plain nozzle and pipe into 10cm lengths onto the prepared baking trays, leaving about 2cm in between each one. Bake for 10 minutes, reduce heat to 180C, prick bases with a skewer and bake until golden and dry ( about 20 minutes). Place on a wire rack to cool.
To make the vanilla custard cream filling, whisk the egg yolks, sugar and flour in a bowl until combined. Place the milk in small saucepan over a medium high heat and bring to just below boiling point. Remove the saucepan from the heat and reduce the heat to low. Gradually whisk hot milk into the egg yolk mixture until combined before returning the mixture to the saucepan. Cook, whisking constantly, over medium heat until thickened and the mixture resembles a thick custard. Strain through a fine mesh sieve into a bowl and cover with plastic wrap to prevent skin forming and refrigerate until cold. Whisk the cream and vanilla seeds with electric mixer until peaks form. Fold the cream into the cold custard and spoon into a piping bag.
For the chocolate glaze melt the chocolate and butter in a small bowl over a pan of barely simmering water and stir until smooth. Cut each eclair in half and spread the top half of each eclair with chocolate mixture, sprinkle with coconut and allow to set. Just before serving, pipe the custard cream filling into bases and replace chocolate tops.
If you want to skip the pastry cream fill the eclairs with whipped cream flavoured with vanilla seeds. A dollop of strawberry jam would be a delicious addition too.