Line a 9 x 9 baking dish with parchment paper or grease lightly with butter.
In a large mixing bowl, combine the melted butter and the sugars.
Add the eggs, salt, and vanilla and whisk just until incorporated.
Sift in the cocoa powder and flour. Mix just until combined.
Add the mini eggs and mix one more time.
Pour batter into the prepared pan and bake for 40-45 minutes or until a toothpick or skewer comes out mostly clean with no crumbs or super wet batter and the center doesn't jiggle.
Video
Notes
Don’t overmix the batter. After adding the flour and cocoa, whisk just until combined. Overmixing can make the brownies tough instead of fudgy. Overbaking can make them dry.I like to put the mini eggs in a sturdy ziptop bag and smash them with a rolling pin. The more inexpensive bags come apart. You can also do this in a food processor or mini chopper.Grease or line the pan with parchment paper. It makes it way easier to lift the brownies out and slice clean squares.Let them cool completely before cutting. Brownies keep baking a little as they cool and firm up. If you cut them too soon, they’ll fall apart.Storage: Once completely cooled, store the Mini Egg brownies in an airtight container at room temperature for up to 4-5 days. They stay soft and fudgy. If your kitchen is really warm, you can refrigerate them.