Preheat the oven to 350° and line an 8x8 pan with parchment paper. Spray lightly with pan spray and set aside.
Use a microwave to gently melt the butter and chopped chocolate in 30-second intervals until fully melted and combined.
Whisk in the sugar, vanilla, and salt. Whisk in the eggs one at a time until just combined. Add the cocoa powder and cornstarch until fully combined and no lumps remain.
Pour the batter into the prepared baking pan and sprinkle the chocolate chips or chunks over the top. Bake for 25-30 minutes, until the center is just set and a toothpick comes out clean. Be careful not to overbake it, it should just barely be baked through.
Allow the brownies to cool completely before removing them from the pan and slicing.