Preheat oven to 350° F. Grease or spray a 10" or 12" tart pan or pie plate or cast iron skillet.
Combine butter in mixer, whipping for 1 minute. Add sugars and mix on medium-high for 1-2 minutes, 2-3 minutes if using all-natural cane sugar. Add vanilla and combine. Add eggs one at a time, mixing until just combined.
In a medium bowl stir together flour, baking soda, baking powder and salt. Add flour mixture to creamed butter mixture, mix on low. Spoon dough into prepared tart pan and smooth out as much as possibly, pressing evenly. Place parchment paper or wax paper over top and smooth with hands.
Bake for 9-11 minutes, then remove from oven (closing door) and place on cookie sheet. Quickly, press Cadbury Eggs (or other candy coated chocolates) in desired pattern on top of half-baked cookie. Return to oven and bake for an additional 7-11 minutes until lightly browned on top. I prefer mine a little less baked, crisp edges and chewier center. Cool completely on cooling rack and slice like you would a pie.
Make ahead 1 day and keep refrigerated until ready to serve, remove from refrigerator 30-60 minutes prior to serving. May be frozen for up to 1 month, bring to room temperature before serving.