Preheat oven to 350°F. Coat a springform pan with non-stick cooking spray and set aside.
With a mixer cream together the butter and sugar until light and fluffy. Add the egg, egg yolk, and vanilla extract and continue mixing until fully incorporated.
In a separate mixing bowl, whisk together flour, salt, baking powder, baking soda and cornstarch.
Gradually mix dry ingredients into bowl of wet ingredients. Continue mixing until you have a dough consistency.
Stir in candy and sprinkles by hand.
Press the dough into the springform pan pressing more M&Ms onto the top as desired.
Bake for 20-25 minutes or until the cookie is light golden brown on top. Let it cool completely before frosting.
FOR THE FROSTING.... beat butter until smooth, then slowly beat in powdered sugar until crumbly. Add salt and vanilla, and1 teaspoon of heavy whipping cream. Beat until smooth, adding another teaspoon of heavy whipping cream to get a spreadable texture.
Remove the cookie cake from the springform pan and place onto a serving plate.
Pipe the frosting around the edge of the cookie cake and top with pastel sprinkles.