• Preheat oven to 350°F.
• Spray the sides of a 9 inch round cake pan with cooking spray. Line the bottom of the pan with a round of parchment paper. Place two long strips of parchment paper in an X shape across the inside and up the sides of the pan; these will be used for helping to lift the cake out of the pan. Set aside.
• Place the Cadbury mini eggs in a plastic food storage bag. Seal the bag, place it on a sturdy surface, and use a rolling pin or mallet to coarsely crush the candies. Set aside.
  1 cup Cadbury mini eggs

Make the cookie cake.

• In a medium bowl, add flour, cornstarch, baking soda, and salt and whisk to combine. Set aside.
  2 cups all-purpose flour, 2 teaspoons cornstarch, ¾ teaspoon baking soda, ½ teaspoon fine sea salt
• In the bowl of a stand mixer (or in a large bowl using an electric hand mixer), cream the butter and both sugars for 3 minutes until light and fluffy.
  ¾ cup unsalted butter, 1 cup brown sugar, ¼ cup granulated sugar
• Add egg, egg yolk, and vanilla and beat for 1 minute or until well blended.
  1 large egg plus 1 large egg yolk, 1 Tablespoon vanilla extract
• With the mixer running on slow speed, slowly add the flour mixture and blend just until combined.
• Add the crushed candies, chocolate chips, and sprinkles and stir just until blended.
  1 cup Cadbury mini eggs, ¼ cup chocolate chips, ¼ cup pastel jimmies (sprinkles)
• Scoop the dough into the prepared pan and use a spatula to spread the dough evenly around the pan without compacting it.
• Bake the cake for 28-30 minutes or until the top is lightly golden and the cake is evenly puffed. Remove the cake from the oven and carefully run a cake tester or butter knife around the edge of the pan to keep the cake from sticking to the pan as it cools. Let the cake cool completely in the pan on a wire rack.

Make the frosting.

• In the bowl of a stand mixer using the paddle attachment (or in a large bowl using a handheld electric mixer), beat butter on medium-high speed until smooth.
  4 Tablespoons unsalted butter
• Add vanilla and salt and beat on medium-high speed until blended.
  ½ teaspoon vanilla extract, 1 tiny pinch fine sea salt
• Add sugar slowly, beating well on medium speed and scraping the bowl often. When all sugar is added the frosting will be slightly dry. 
  1 cup confectioners' sugar
• Add milk and beat on high speed until the frosting is light and fluffy.
  1 ½ teaspoons whole milk

Decorate the cake.

• Remove the cooled cake from the pan. Run a cake tester or butter knife once again around the side of the pan to loosen any stuck edges. Lift up on the strips of parchment to loosen and lift the cake from the pan. It may be helpful to have someone help you lift the cake out of the pan using the strips. Place the cake on a cake plate.
• Fit a piping bag with a large open star piping tip (I used Wilton 8B) or the piping tip of your choice. Pipe dots of frosting around the edges of the cake.
• Sprinkle a few extra jimmies over the frosting.
• Slice and serve!