• Preheat oven to 350°F.
• In a medium-sized microwave-safe bowl, melt the unsalted butter (¾ cup, i.e. 1 ½ sticks) for 20-30 seconds until fully melted. 
• Add in granulated sugar (½ cup) and light brown sugar (¾ cup) and use a spatula to stir until combined (*do not use a mixer!). Add in egg (1 large), egg yolk (1 large), and vanilla extract (2 teaspoons). Mix until combined.
• Stir in baking soda (1 teaspoon), baking powder (¼ teaspoon), and salt (½ teaspoon). Add all-purpose flour (2 cups) and mix until all the flour disappears into the dough. Stir in crushed Cadbury eggs (1 cup) and semi-sweet chocolate chips (1 cup). Immediately proceed to the next step.
• Make a DIY foil pan, spray it with nonstick cookie spray, and place on parchment-covered baking sheet. You can also bake this in a 10" round cookie cake pan (keep an eye on baking, as it may only need 16-18 minutes in the metal pan).
• Bake at 350°F for 18-20 minutes (for the DIY foil pan) until edges and top are lightly golden brown. Do not overbake or your cookie cake will not be soft. You want it to be lightly brown on the top and look slightly underbaked (my oven was perfect at 20 minutes). Remove from oven and let cool.
• Allow the cookie cake to cool completely before removing the foil ring and decorating.
• Slice into 12 -16 slices, then enjoy! To keep fresh, cover with saran wrap or place cake in airtight continer for up to 3 days (after that it starts to dry out).

Frosting:

• Beat the softened unsalted butter (¼ cup, i.e. ½ stick) on medium-high speed for 2 minutes until smooth. Turn the mixer to low speed then add the powdered sugar ¼ cup at a time (1 ¼ cups total). You may need to stop and scrape down the sides.
• Add vanilla extract (1 teaspoon) and a pinch of salt and turn mixer to medium speed. One tablespoon at a time, add milk (1-2 tablespoons). Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency.
• Mix in green food coloring.
• Use a large star piping tip to pipe on border (to look like the grass), then decorate with Cadbury eggs. If you don't have a large star tip, use this fun hack to pipe on grass instead or just pipe on large circles with a ziploc bag!
• This will make just enough frosting to pipe a border along the edges. If you would like more frosting to cover the top or to write words on top, simply double the frosting recipe!