Some French petits fours are quite elaborate but the ones consisting of choux pastry with a pastry cream filling can be managed even by home bakers. Start by making the custard so it has time to chill.
In a saucepan, bring milk and vanilla to the boil. Meanwhile combine eggs, sugar and flour in a medium bowl and mix well. Gradually add hot milk, whisking constantly. Pour this mixture back into the saucepan and bring to the boil once again, then reduce heat to low; simmer, stirring constantly with wire whisk, until cream thickens.
Divide custard into 2 bowls and add the cocoa to one bowl, stir to dissolve. Cover and let cool in refrigerator.
Preheat oven to 210 C / Gas 7 and grease two baking trays.
In a large saucepan bring water, sugar, salt and butter to the boil. Add flour all at once and stir with a wooden spoon. Remove from heat and add eggs one at a time, mixing well after each egg.
Pipe small mounds of pastry (round or elongated), using a pastry bag, on prepared baking sheets.
Bake for 25 minutes; pastries should be golden and puffy. Let cool on rack.
Pierce tops of pastries with a knife and fill with chilled vanilla or chocolate cream using a pastry bag with a fine tip.
In a small bowl mix enough lemon juice with a bit of icing sugar for a smooth, thick icing. Add a few drops of food colouring for desired colour, if using.
Brush icing on top of pastries and place on serving plate or platter. Refrigerate for 2 hours before serving.