Cadbury Creme Eggs Stuffed Chocolate Chip Cookies

Cream the butter and sugars together on medium speed until fluffy and light in color in stand mixer. Mix in egg and vanilla. On low speed, mix in flour, cornstarch, baking soda and salt. Stir in the m&ms. Chill dough for 1 hour.
After the dough is chilled, line two cookie sheets with silpat mats. Preheat oven to 350°F. Cut eggs in half with a sharp knife.
Take about four tablespoons of dough and flatten dough with hands. Place egg inside dough. Make sure to wrap the dough around the egg so that the egg's open yolk portion faces is on its side (so that when the person bites in, the yolk is exposed). Make sure dough is completely sealed around egg and dough is thick enough so that the inside won't seep through. Press a few additional m&ms on surface of dough ball if desired.
Bake the cookies for 10-11 minutes, until barely golden brown around the edges. Remove and let cool on the cookie sheets. Do not try to remove before they are finished cooling or cookies may break.