Preheat the oven to 200˚C (180˚C fan oven, 400˚F, gas 6). Line a large baking tray with baking parchment. Put the butter in a saucepan with 150ml (1/4 pint) water.
Heat gently until melted then bring to the boil. Turn off the heat and immediately mix in the flour, beating well until thick, glossy and smooth – the mixture should form a soft ball of dough in the saucepan. Transfer to a heatproof bowl and cool for 10 minutes.
Gradually whisk in the eggs to make a thick, sticky mixture. Spoon into a piping bag fitted with a 1.5cm (2/3 inch) wide plain nozzle and pipe 16 buns onto the baking tray approx. 3cm (1 ¼ inch) diameter. Bake for 25 minutes until puffed up and golden.
Carefully pierce the side of each bun with the point of a sharp knife – take care as there will be a release of hot steam from the centre. Put back on the baking tray and pop in the oven for a further 8-10 minutes until richly golden and very crisp. Transfer to a wire rack to cool completely.
Put the cream, icing sugar and Vanilla Paste in a large bowl and whip the cream until softly peaking.
When ready to serve, slice each bun through the middle using a serrated knife. Generously spoon the cream into the bases of the choux buns or spoon the cream into a piping bag fitted with a 1.5cm (2/3 inch) plain nozzle and pipe into each. Place the tops on each choux bun to sandwich the cream filling.
To decorate, break the Extra Dark Chocolate into a heat proof bowl over a pan of barely simmering water, add the butter. Heat gently, stirring occasionally, until melted together and smooth. Remove from the heat and set aside for about 20 minutes to cool to a thick, coating consistency.
When the Chocolate starts to thicken, spoon over the top of each bun. Your choux buns are now ready to serve and enjoy!