Preheat oven to 350°F. Line a 9x9” pan with foil and spray with cooking spray.
Place the two baking chocolates and butter in a large, microwave safe bowl. Heat on HIGH power for 2-3 minutes, stirring every 30 seconds, until chocolate is smooth. Stir in sugar. Add eggs, vanilla, and cocoa and stir well. Add flour and stir carefully. Stir in mini chocolate chips.
Spread brownie batter in prepared pan. Bake for 25-30 minutes, until a toothpick comes out with just a few crumbs stuck to it.
While the brownies are baking, make the butterscotch sauce. Melt the butter in a medium saucepan over medium heat. Add the brown sugar and heavy whipping cream and stir until it boils. Once it boils, let it sit without stirring, and cook for 3 minutes. Remove from heat and stir in vanilla and 1 1/2 teaspoons salt. Transfer to a jar and let it cool on the counter until the brownies are done. (Makes about 1 3/4 cups, you won’t use it all on the brownies.)
When brownies are done, remove them from the oven and immediately top with 3/4 cup of the butterscotch sauce. Let cool completely before slicing. This will take several hours, but you can also chill them for faster cooling. The sauce doesn’t harden but does solidify slightly. The brownies are sticky and gooey. Sprinkle with a few pinches of sea or kosher salt before slicing and serving.
Store brownies in an airtight container for up to 4 days. Extra butterscotch sauce can be kept sealed for up to one week. It will harden in the refrigerator (and semi-solidify on the counter) but if you microwave it it becomes liquid again. Use it on anything you’d put caramel on!