Preheat your oven to 350°F (175°C). Prepare a 9-inch round cake pan by spraying it with cooking spray and lining the bottom with parchment paper.

Crush the Cadbury Mini Eggs in a sealed plastic bag using a rolling pin or mallet.

In a mixing bowl, whisk together the all-purpose flour, cornstarch, baking soda, and fine sea salt.

In a stand mixer, beat the room-temperature unsalted butter, brown sugar, and granulated sugar on medium speed until light and fluffy.

Add in the large egg, extra yolk, and vanilla extract, mixing until combined.

Gradually add the dry ingredient mixture to the wet ingredients, mixing just until combined.

Fold in the crushed Cadbury Mini Eggs, chocolate chips, and optional pastel jimmies.

Pour the cookie dough into the prepared cake pan, spreading it evenly, and bake for 28-30 minutes.

Let the cookie cake cool in the pan for about 10 minutes, then transfer it to a cooling rack.

For the frosting, beat softened butter until smooth, add confectioners' sugar, vanilla extract, and salt, then mix with milk until fluffy.

Once the cake is cool, frost the top generously with buttercream frosting and sprinkle additional pastel jimmies.

Slice the cookie cake into wedges and serve to your family and friends.