Preheat your oven to 350°F. Line an 8×8-inch baking pan with parchment, leaving an overhang for easy lifting.
Chop the baker’s chocolate and place it in a microwave-safe bowl. Melt it in 20-second bursts, stirring between each burst until smooth. Let cool slightly.
In a large bowl, combine the melted butter and sugar. Whisk by hand for 2–3 minutes until well combined and glossy.
Add the eggs and vanilla. Whisk vigorously for about 5 minutes until the mixture is lighter in color and has increased in volume.
Pour in the melted chocolate and mix until uniform.
Sift the flour, cocoa powder, and salt over the wet ingredients. Fold with a rubber spatula just until no streaks of flour remain.
Fold in 1 cup mini eggs gently. Transfer batter to the lined pan and spread evenly. Press a few extra mini eggs on top.
Bake for 28 minutes, or until the edges are set and the center is just set. Cool completely in the pan before slicing into squares.