For the posset, place the double cream and the sugar into a large pan over a low heat and bring to the boil slowly. Boil for three minutes, then remove from the heat and allow to cool.
Add the lemon juice and zest and whisk well.
Pour the lemon cream mixture into six large serving glasses and refrigerate for three hours.
Preheat the oven to 180C/355F/Gas 4.
For the shortbread, sift the icing sugar, flour and cornflour together into a bowl and add the ground almonds.
Transfer the flour mixture to a food processor. Add the butter and pulse until there are no visible lumps of butter.
Add the almond essence. Pulse again, then turn the mixture out onto a lightly floured surface and bring together to form a smooth dough.
Grease a muffin tray with butter.
Divide the dough up and roll into small balls. Place the balls into the muffin cups, flattening the tops slightly with your fingers. The dough should come about one third of the way up the side of each muffin cup to give a nice proportion to the finished biscuit.
Transfer to the oven and bake the shortbreads for 8-12 minutes, until they are a light golden colour.
Remove from the oven, allow to cool slightly, then, using your thumb, make a small indentation into the top of each biscuit.
Let the shortbreads cool for a few minutes, then turn the mould over and tap the shortbreads out. (Be gentle, as the biscuits are fragile while they are still warm.)
When all the shortbreads are baked and cooled, dust the tops with icing sugar.
Fill the indentations in the biscuits with lemon curd.
To serve, place the lemon possets onto plates with a shortbread alongside.