Preheat your oven to 350°F (175°C) to ensure even baking.
Line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease the paper.
In a mixing bowl, combine room temperature cream cheese, 1 large egg, 1-1/2 tablespoons all-purpose flour, and 1/3 cup granulated sugar. Whisk until smooth and creamy. Gently fold in the picked mint leaves. Set aside.
Using a double boiler or microwave-safe bowl, melt 6 ounces dark chocolate and 1/2 cup unsalted butter. Stir until smooth and let cool slightly.
In a large bowl, whisk together 3 large eggs, 1/3 cup light brown sugar, and 1/3 cup granulated sugar until frothy. Add melted chocolate mixture and 1 teaspoon vanilla extract; mix until combined.
Sift together 1/2 cup plus 1 tablespoon all-purpose flour, 1/3 cup unsweetened cocoa powder, and 1/8 teaspoon kosher salt. Gradually fold into the chocolate mixture until just combined. Avoid overmixing.
Pour half of the brownie batter into the prepared pan, spreading evenly. Spoon the mint cream cheese mixture over it, then dollop remaining brownie batter on top. Use a knife or toothpick to gently swirl together.
Bake in preheated oven for 30-35 minutes until a toothpick inserted in center comes out with a few moist crumbs.
Allow brownies to cool in pan for at least 15 minutes. Lift out using parchment paper overhang, transfer to wire rack to cool completely, then slice and serve.