Preheat oven to 350°F and line a 9x13 pan with parchment paper.
In a large microwave-safe bowl, combine 1 stick melted butter, granulated sugar, and brown sugar. Microwave for 2 minutes, stirring every 30 seconds, until smooth and glossy.
Let mixture cool slightly. Add applesauce, cornstarch, vanilla, and baking soda. Stir until fully combined.
Add cocoa powder and flour, mixing until almost combined.
Fold in remaining melted butter until the batter is smooth and fully combined.
Spread batter evenly into prepared pan.
Bake for 20 minutes, then cool completely.
Heat heavy cream for about 1 minute. Add chocolate chips and let sit for 5 minutes.
Stir until smooth, then let cool for about 30 minutes until slightly thickened.
Spread ganache over cooled brownies and top with vegan rainbow chocolate chips.