Mix together the butter and sugars (I use an electric whisk for about 1 minute, if mixing by hand it will take a little longer).
Add the egg, syrup, and vanilla and mix for 2 minutes until smooth.
Sift in flour and bicarbonate of soda.
Add the crushed mini eggs and fold together until it forms a dough.
Cover and place in the fridge for an hour.
Remove the dough from the fridge and preheat the oven to 180 degrees C (fan assisted).
Roll into 20 equal balls and place on baking sheets (I put 6 on a sheet and bake in 2 batches, leaving the leftover dough in the fridge until I need it).
Press down slightly into thick rounds, about 5mm-1cm.
Press in the remaining mini eggs gently.
Bake in the oven for 10-12 minutes.
Remove from the oven and cool on a wire rack.
Notes
Cookies are cooked when the edges are set. If you like a crunchy cookie, bake until the middles are set too, around 12 minutes. If you prefer them chewy, remove them from the oven when the edges are firm, but the centres are still soft, for me, this is around 10 minutes. Remember, they will carry on cooking, and firm up more as they cool.
You may need to mix for a little longer if mixing by hand, but try to avoid overmixing, stop as soon as the mix is even at every step.
I don't find this dough spreads too much, as long as it's well cooled, so don't worry about pressing them down. If yours do spread, press down less next time.
Even after time in the fridge, the dough is quite sticky, so if you'd rather not get messy, use an ice cream scoop to create your dough balls.