Preheat the oven to 350°F. Line a 9x9-inch pan with parchment paper.
In a large bowl, whisk together 1 cup melted butter, 3 eggs, and 2 teaspoons vanilla until smooth.
Stir in 1 cup granulated sugar and 2/3 cup brown sugar until fully combined.
In a separate bowl, whisk together 3/4 cup flour, 3/4 cup cocoa powder, and 1/2 teaspoon salt.
Fold the dry ingredients into the wet ingredients just until combined (do not overmix).
Gently stir in 1/2 cup chopped semi-sweet chocolate and 1 cup of the chopped Mini Eggs.
Spread the batter evenly into the prepared pan.
Bake for 25–35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
Immediately press the remaining 1/2 cup Mini Eggs into the top of the warm brownies.
Cool completely before slicing and serving.