Preheat the oven to 325° Grease an 8x8 pan with butter & parchment. Roughly chop the entire bag of mini eggs. NOTE: smashing the eggs with the flat side of knife (like you would a clove of garlic) is easiest.
Whisk Eggs & Sugar & Wet Ingredients: In a medium bowl, add 2 eggs and 1 egg yolk, brown and white sugar. Whisk together until combined. In a microwave safe bowl, melt butter & semi sweet chocolate in 30 second increments. Stir to combine and pour the chocolate butter mixture into the eggs and sugar. Whisk until combined. Add vanilla extract & stir.
Whisk Dry Ingredients: In a large bowl, add flour, cocoa powder & salt. Whisk until combined.
Combine Both Wet & Dry: Add dry ingredients to wet & whisk until combined. Fold in the last dry bits with a rubber spatula and add 3/4 cup of the smashed Cadbury eggs.
Add the batter to your greased baking dish. Spread it evenly. Sprinkle the rest of the cadbury eggs over the top (you may have a little bit left) I like to sprinkle the shattered shell pieces over the top as well.
Bake for 30 minutes or until the brownies are set and a toothpick comes out almost clean with maybe a few moist crumbs.