Begin by setting your oven to 350-degrees F. This steady, moderate temperature is ideal for brownies. Then, generously grease your 13x9x2-inch baking pan. A well-greased pan ensures your brownies will release easily, preserving their perfect shape and texture.
In a medium, microwave-safe bowl, combine the butter and chocolate. Microwave for 1 minute on high. After this initial heat, give the mixture a thorough stir. The residual heat will help melt the chocolate evenly. If needed, continue microwaving in short 30-second bursts, stirring after each interval. The goal is a velvety, homogenous mixture - the base of your brownie's flavor.
Once your chocolate mixture is smooth, stir in the sugar. Then, add the eggs and vanilla, gently mixing them in. Be cautious not to overbeat at this stage; we want to avoid incorporating too much air into the batter, which can affect the brownies' texture.
Gently fold in the flour. This step is crucial: fold until just combined to keep the brownies dense and fudgy. Overmixing after adding flour can create a cake-like texture, which isn’t what we're aiming for in a classic brownie.
Pour the batter into your prepared pan, smoothing it into an even layer. Bake for 30 to 35 minutes. To test for doneness, insert a toothpick into the center. If it comes out with a few moist crumbs, the brownies are perfectly baked. Let the brownies cool completely in the pan set on a wire rack. Cooling is essential as it sets the brownies and makes the addition of the next layer easier.
In another microwave-safe bowl, combine marshmallows and milk. Microwave for 2 minutes, then stir until smooth. The marshmallows should be fully melted into a creamy consistency. Add peppermint extract and green food coloring, adjusting to your preferred intensity of flavor and color. Chill this mixture until slightly thickened, ensuring it's firm enough to hold up when folded into the whipped cream.
Fold the whipped cream into your chilled marshmallow mixture, creating a light, fluffy layer. Gently spread this over your cooled brownie base. Freeze the combined dessert to solidify the layers, turning it into a cohesive, sliceable treat.
Before serving, transfer the Brownie Tortoni from the freezer to the refrigerator for about 30 minutes. This resting period makes slicing easier and improves the eating experience. Finally, garnish with drizzed chocolate for a delicious finish.