Preheat oven to 325ºF. Line an 8x8" baking dish with parchment paper, leaving a bit of overhang at the sides.
Melt butter in a large microwave-safe bowl in the microwave on high power for about a minute. Use a fork to stir in cocoa powder until completely blended, then fork-whisk in sugar, eggs, vanilla, salt and flour. Stir in chopped chocolate.
Use a spoon to spread the stiff batter into the parchment-lined baking dish, then sprinkle mini eggs evenly over top. Bake for about 30 minutes, until centre is firm but still has a bit of give. Let cool at least 10 minutes in pan before lifting out brownies by the parchment paper.