Preheat your oven to 180°C / 350°F. Line a square 9×9 inch (23×23 cm) cookie pan with baking parchment.
In a large bowl, cream the softened butter together with the light brown sugar using a wooden spoon, until it is light and fluffy.
Add the egg (at room temperature) and vanilla bean paste/extract then mix well.Make sure to use an egg which is at room temperature to avoid the mixture curdling, you are aiming for a nice even and well mixed texture. Add the flour, cornflour, cream of tartar and a pinch of salt, then mix together with a wooden spoon until the dough starts to come together.
You might need to finish mixing with your hands, once all combined the dough should be coming away from the side of the bowl but still light and fluffy.
Add the Mini Eggs, holding a few back to add on top, and mix them into the dough.
Press the dough into the lined cookie pan, note that it will be quite thin but don’t worry as it will rise a little as it cooks, and add the remaining Mini Eggs on top.
Bake for 18 minutes at 180°C (350°F), until starting to go golden on top and around the edges, with a dry top. Remove the pan from the oven and allow to cool.
Cut into 9 pieces and enjoy!