Preheat the oven to 350°F (175°C).
Line an 8 × 8-inch baking pan with parchment paper and lightly grease the exposed sides.
Whisk the melted butter, granulated sugar, and brown sugar together.
Add the eggs one at a time and mix just until combined.
Stir in the vanilla.
Sift in the cocoa powder, flour, and salt.
Fold gently until just combined.
Fold in the chocolate chips or chopped chocolate.
Spread the batter evenly into the prepared pan.
Bake for 22–27 minutes, until the center is set but still fudgy.
Cool completely in the pan.
Place the chopped chocolate for the ganache in a heat-safe bowl.
Heat the cream until steaming but not boiling.
Pour the hot cream over the chocolate and let it sit for 2 minutes.
Stir gently until smooth and add the butter if using.
Spread the ganache over the cooled brownies.
Scatter the mini candy-coated chocolate pieces over the soft ganache.
Refrigerate for 30–45 minutes.
Lift the brownies from the pan using the parchment paper.
Slice into 16 pieces and serve.