Preheat the oven to 350 degrees. Grease an 8x8 nonstick baking pan and line it with parchment paper, leaving at least 2 inches of paper hanging over the sides.

In a medium sized bowl, add melted butter, granulated sugar and brown sugar and mix with your hand mixer on medium speed.

Add eggs one at a time, mixing after each addition to incorporate. Mix well!

Mix in oil and vanilla extract.

Combine flour, cocoa powder, baking soda if using and salt in a bowl. Reduce the speed of the mixer.

In intervals, add flour mixture to wet mixture until all ingredients are incorporated.

Add brownie batter to the prepared baking pan. Bake for 30 minutes, or until a toothpick inserted into the center comes out just barely clean.

Allow the brownies to cool in the pan for about an hour, or until as close to room temperature as possible.

Place the heavy cream in a medium-sized microwave-safe bowl. Microwave in 15 second increments until cream is very hot (not boiling). Stir in between increments.

Add the chopped chocolates to the cream. Let sit for a minute, then whisk until a smooth chocolate mixture forms. Stir in the butter until melted.

Pour the chocolate ganache over the top of the brownies. Use a flat spatula to spread the chocolate into an even layer. Cool for 5 minutes. Sprinkle the rainbow chocolate chips on top.

Cover and chill the brownies until set, about 1 hour. Use the parchment to lift the brownies out of the pan. Cut into squares and serve.