Move oven racks as close to the top as you and preheat the oven to 350°F.
Lightly grease an 8×12-inch baking pan with cooking oil spray. Line with parchment paper; set aside.
Combine melted butter, oil, and sugars together in a medium-sized bowl. Whisk well to combine. Add the eggs and vanilla; beat until lighter in color (about another minute).
Sift in flour, cocoa powder, and salt. Gently fold the dry ingredients into the wet ingredients until just combined. Do not over-mix if you do it will affect the texture of your brownies.
Fold in 1 cup of the mini eggs.
Pour batter into the prepared pan, smoothing the top out evenly, and top with the remaining 1/2 cup mini eggs.
Bake for 25-30 minutes on the top rack, until the center of the brownies no longer jiggles and is set to the touch.
