Preheat oven to 350°F/180°C. Line baking sheets with parchment paper or silicone mats.
Melt butter in a small saucepan over medium-low heat. Once melted, leave to cool for 20 minutes.
In a large bowl, whisk together flour, baking soda, baking powder, cornflour and salt.
In a separate bowl, add the cooled melted butter, sugars and vanilla and whisk for about 5 minutes or until light and fluffy.Add the egg and egg yolk and continue whisking for 1-2 minutes until the batter looks smooth and creamy.
Stir in the flour until it's just combined with the sugar mixture, being careful not to overmix.
Fold in the chocolate chips/chunks and the mini eggs until just blended into the cookie dough.
Refrigerate the cookie dough for 20 minutes to firm it up.
Scoop out into balls (about 2 tbsp of dough each) and place on your prepared baking sheet. Leave at least 2 inches between each cookie to allow for spreading.
Bake for around 10-12 minutes for soft cookies, or around 13-15 for crisper cookies.
Once baked, allow cookies to cool on the tray for 5 minutes before transferring to a cooling rack to cool completely.