To prepare the cake, preheat the oven to 200C/180C/gas 4. Line 2 baking trays with baking parchment. Line a 25cm square cake tin with baking parchment on the base and brush the sides with melted butter.
Place the water and butter in a saucepan and bring to the boil. Sieve the flour.
Lower the heat, add the sieved flour to the butter and water. Whisk very briskly until the pastry comes away from the sides of the saucepan.
Remove from the heat and add the eggs, a little at a time, whisk very well until a soft glossy pastry forms.
Spoon into a piping bag fitted with a plain nozzle.
Pipe 5cm pastry out onto the sheets and place into the oven to bake for about 20 minutes until risen and golden.
Remove from the oven, make a hole in the top of each of the éclairs, then turn them over and return to the oven for about 3 to 4 minutes.
To prepare the filling cream, whisk the egg yolks and rich dark sugar together in a mixer until light and fluffy.
Pour the milk, cocoa powder, cornflour and shot of espresso into a saucepan and heat over a medium heat.
When the machine is still running, pour the hot milk into the egg sugar mix steadily and then return the mixture to the saucepan, heating over medium heat, while stirring all the time until a very thick sauce forms.
Leave to cool completely’ then fold in the whipped cream and spoon into a piping bag fitted with a plain nozzle.
To assemble the cake, melt the chocolate in a shallow bowl.
Pipe the filling into each éclair, turn over and dip the other side in the melted chocolate.
Invert and leave to set for a short while before decorating with edible flowers.