Add the butter cut into cubes to a pan set over a low heat - allow it to melt, then leave it to cool slightly.
Add the cooled butter to a large bowl, and add the white and brown sugar.
Use a hand mixer to mix everything together until you get a smooth paste.
Add the egg and egg yolk, and mix again, the mixture will double its volume and become a lighter colour.
In go the sifted flour, bicarb of soda, vanilla extract - use a spatula to mix everything into a dough.
Throw in the chocolate eggs, and gently fold them in using a spatula.
Leave the mixture to rest for 5-10 minutes so that the dough can thicken a bit, it can be pretty soft at this stage.
Line 2 large baking trays with non-stick baking paper or grease and flour a non-stick tray.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Use your hands to shape balls out of the dough - I arranged only 6 balls onto each baking tray to allow plenty of room for spreading - so work in batches and with different baking trays.
Bake for about 10-11 minutes, or until the edges are slightly darker, and the cookies are still a bit puffed up.
Leave to cool on a hard surface before transferring onto the cooling rack.
Repeat with the remaining dough.