Preheat the oven to 350F.
In a large mixing bowl of a stand mixer, cream the butter until it's nice and silky about 2 minutes. Then add the brown sugar and granulated sugar and continue to mix until it is smooth about 2 minutes. (If you don't have a stand mixer, you can use a hand mixer or some good 'ol elbow grease).
Next mix in the egg, egg yolk and vanilla, beating until smooth and well combined.
In a separate bowl whisk together flour, baking soda and salt; then fold the mixture into the wet ingredients with a spatula. Lastly fold in the crushed mini eggs.
Line a baking sheet with parchment paper and scoop the dough with a small ice cream scoop or a loaded tablespoon. Roll the dough for a few seconds in your hand and pop down on the baking sheet leaving about 2 inches of space between cookies and the edge of the pan as the cookies will spread a bit. You will most likely have to do it in 2 or 3 batches depending on the size of you cookie sheet.
Bake for 10-12 minutes or until edges are just starting to turn a golden brown. I check the first batch after 10 minutes and then pop them back in if I need to. In my oven it is 12 minutes exactly. But they are best when they are a little undercooked as they will cook a bit more as they cool and remain nice and soft.
Transfer to a baking rack to cool completely and then get them in your belly!