In the bowl of an electric stand mixer fitted with a paddle attachment, cream the butter and sugar for three minutes minimum until pale in colour, scraping down the sides of the bowl halfway.
In a separate bowl, sift the flour, cornflour, baking powder, baking soda and salt.
Scrape down the mixing bowl sides and add the egg and vanilla extract, and beat until combined.
Gradually add the dry ingredients (flour mix), and 100g of mini eggs chopped into small pieces and the white chocolate, then mix until combined.
Line a baking tray with parchment paper and preheat the oven to 180C/350F. Using a cookie scoop or a teaspoon, scoop balls of cookie dough into inch sized balls and place them on a baking tray and place them in the freezer for 20 minutes (or fridge for 40 minutes).
Remove the cookie dough balls from the freezer and place them on two lined baking trays, leaving a good 2 inches between the balls.
Bake for 7 to 8 minutes until the edges are just turning golden. My preferences are soft, chewy cookies but if you prefer crispier cookies, then add an extra minute or so to the cooking time. Remove the tray from the oven; whilst still hot, sprinkle the remaining larger pieces on mini eggs onto the cookies, pushing them in gently. Using a large cookie cutter, place it over a cookie and twist it around, pushing the edges of the cookie gently in to help form it into a round shape. Leave on the baking tray for 15 minutes to cool and then transfer to a wire rack to cool further. Serve immediately.