Preheat oven to 350°F. Line an 8x8 pan with parchment paper.
Melt butter and stir in sugar. Whisk in eggs, egg yolk, and vanilla. Stir in melted chocolate.
Fold in sifted flour, cocoa, salt, and baking powder until just combined. Do not overmix.
Bake 20 to 24 minutes until center is just set with moist crumbs on the toothpick. Cool 30 minutes.
Make ganache by pouring hot cream over chopped chocolate and corn syrup. Rest 2 minutes then stir smooth.
Pour ganache over cooled brownies, spread to edges, and scatter rainbow chips on top. Refrigerate 2 hours before cutting.