Preheat the oven to 190°C/170°C fan and line an 8inch/20cm square tin with greaseproof baking paper.
In a large bowl, cream together the butter with the sugars for 2-3 mins using a stand mixer or electric whisk.
Add the egg and vanilla extract and beat again until combined.
In a separate bowl mix together the flour, baking powder, bicarbonate of soda and salt.
Once the dry ingredients are mixed, add these into the butter and sugar mixture. Mix until combined.
Chop up the Mini Eggs bar into small pieces and add them to the cookie dough, mixing until evenly distributed.
Spread the cookie dough across the bottom of your prepared tin, making sure its level and goes right up to the edges.
Bake in the oven for 15-18 mins, until golden brown. Remove from the oven and set aside to cool.
