Grease an 8×8-inch pan with butter, then line with parchment paper, leaving an overhang on all sides. Grease the parchment with butter, then set aside.
Combine the chopped chocolate and 2 tablespoons (15 g) of cocoa powder in a medium heatproof bowl and set aside.
Add the butter to a small saucepan over medium heat and cook until the butter just comes to a vigorous simmer, stirring often. Immediately pour the hot butter over the chocolate mixture and let sit for 2 minutes. Whisk until the chocolate is completely smooth and melted, then set aside.
Preheat oven to preheat to 350°F (180°C).
In the bowl of a stand mixer fitted with the whisk attachment, or using an electric hand mixer, whisk together the granulated sugar, brown sugar, vanilla extract, salt, and eggs for exactly 10 minutes on high speed.
With the mixer on low, pour in the slightly cooled chocolate mixture and mix until smooth.
Sift in the flour and remaining cocoa powder and use a rubber spatula to gently fold until just combined. Fold in about ½ cup of the mini eggs.
Pour the batter into the prepared baking pan and smooth the top with a spatula. Sprinkle the remaining mini eggs on top and bake until a wooden skewer inserted into the center of the brownies comes out fudgy but the edges look cooked through, about 30-35 minutes. The center of the brownies will seem underbaked, but the brownies will continue to set as they cool. Allow brownies to cool completely in the pan.
Use the parchment paper to lift the cooled brownies out of the pan. Cut into 16 bars and serve.
*It’s better to pull the brownies out of the oven early than leave them in too long—if you overbake the brownies, they’ll be dry & tough.