Grease a 20cm square cake tin and line the bottom and sides with baking parchment. Put the flour and sugar into a food processor and blitz until combined. Add the butter with the vanilla and blitz until the mixture resembles sandy breadcrumbs. Pour the mixture into the prepared tin and pack down lightly until level. Chill for 30 minutes.
Preheat the oven to 180°C/160°C fan/gas mark 4. Once the biscuit base is chilled, bake for 30 minutes until just golden, then allow to cool.
To make the caramel layer, put all of the ingredients apart from the micro eggs into a medium heavy-bottomed saucepan over a medium heat. Bring to the boil and stir for 5-6 minutes until thickened and slightly darker in colour. Remove from the heat, stir in the chopped micro eggs and pour over the cooled shortbread. Chill in the fridge for 30 minutes.
Put the chocolate into a heatproof bowl set over a pan of barely simmering water and stir until melted. Don’t let the water touch the bottom of the bowl, otherwise the chocolate might seize. Remove the bowl from the heat and let the chocolate cool for 5 minutes before pouring it over the caramel. Scatter over the whole micro eggs and allow to set in the fridge for 20 minutes, or until the chocolate has just firmed up but is still soft enough to cut with a knife. Remove from the fridge and allow to come to room temperature, then carefully remove the tin and cut the shortbread into 25 squares.