Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang on the long sides for easy lifting. Lightly grease the parchment and sides of the pan with butter or nonstick spray.

In a medium saucepan over low heat, melt 1 cup (226 g) unsalted butter. Remove from the heat and whisk in the granulated sugar and brown sugar until the mixture looks glossy and thick. Let cool for 3–4 minutes so it won’t cook the eggs.

Whisk in the eggs one at a time, beating well after each addition until the batter is shiny and thick. Stir in 2 teaspoons vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, and salt until well combined and no cocoa lumps remain.

Gently fold the dry mixture into the butter-egg mixture just until no streaks of flour remain. Do not overmix to keep the brownies fudgy.

Fold in 1 cup semi-sweet chocolate chips until evenly distributed throughout the brownie batter.

Spread the brownie batter evenly into the prepared pan. Bake for 23–28 minutes, or until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).

Place the pan on a wire rack and let the brownie layer cool completely in the pan. Do not add the mint layer until the brownies are fully cool, or the frosting will melt.

In a mixing bowl, beat 1/2 cup softened unsalted butter until creamy. Add about half of the powdered sugar and mix on low until combined. Add the remaining powdered sugar, peppermint extract, 1/4 teaspoon vanilla, a pinch of salt, and 2 tablespoons cream or milk. Beat for 2–3 minutes until smooth and fluffy, adding more cream or powdered sugar as needed to reach a spreadable consistency.

If desired, add 2–3 drops of green food coloring to the frosting and mix until evenly tinted. Fold in the mini chocolate chips until evenly distributed.

Once the brownie base is completely cool, spread the mint frosting evenly over the surface with an offset spatula or the back of a spoon. Chill the pan in the refrigerator for 20–30 minutes, or until the mint layer is firm to the touch.

In a microwave-safe bowl, combine 1 1/2 cups semi-sweet chocolate chips and 3 tablespoons unsalted butter. Microwave in 20–30 second bursts, stirring after each, until melted and smooth. Let the mixture cool for 3–4 minutes so it is still pourable but not hot.

Pour the slightly cooled chocolate mixture over the chilled mint layer. Tilt the pan or use a spatula to spread it into a thin, even layer over the entire surface.

Refrigerate the brownies for 45–60 minutes, or until the chocolate topping is fully set. Lift the slab from the pan using the parchment overhang. For clean cuts, warm a sharp knife under hot water, dry it, and slice into 24–32 small squares, wiping the knife between cuts.

Serve at cool room temperature for the best fudgy texture. Store leftover brownies in an airtight container at room temperature for up to 2 days (if your kitchen is cool), or refrigerated for 5–6 days. For longer storage, freeze for up to 2 months.