Preheat your oven to 350°F (175°C). Generously spray a 9×13-inch baking dish with non-stick spray and line it with parchment paper, leaving some overhang for easy removal. Set aside.
In a large saucepan, melt the butter and chocolate chips together over medium heat, stirring continuously with a spatula or wooden spoon. Keep stirring until the mixture is completely smooth and glossy, with no chocolate pieces remaining.
Remove the pan from heat and add the granulated sugar and vanilla extract. Whisk until fully combined and the sugar begins to dissolve into the warm mixture.
Sift in the flour to avoid clumps, then stir until all dry spots disappear. Be sure not to overmix, as this can create a tougher texture.
Add the eggs one at a time, mixing until just incorporated after each addition. This will help create a dense, chewy texture without overdeveloping the batter.
Pour the brownie batter into the prepared baking dish, smoothing the top with a spatula to create an even layer. Bake for 23–25 minutes, or until a toothpick inserted into the center comes out mostly clean with only a few moist crumbs.
Remove from the oven and place on a wire rack to cool completely before frosting. Do not frost while warm, as it will melt the mint layer.
