Position a rack in the center of the oven and preheat to 350°F. Grease an 8×8-inch pan and set it aside.

Make the brownie base: place a heatproof bowl over (not touching) a pan of simmering water. Add ¾ cup unsalted butter (12 tablespoons) and 3 ounces unsweetened Baker’s chocolate (3 1‑ounce squares). Stir frequently until melted and smooth. Remove the bowl from the heat and let the chocolate mixture cool completely to room temperature.

Once the chocolate mixture is cool, add 3 large eggs one at a time, beating well after each addition (use a whisk or electric mixer). Add ½ teaspoon pure vanilla extract and 1 ½ cups granulated sugar, and beat until thoroughly combined.

Add the dry ingredients: ½ cup whole wheat pastry flour (or ½ cup additional all-purpose flour, if substituting) and ¼ cup all-purpose flour. Beat or stir just until blended and no streaks of flour remain.

Spread the batter evenly into the prepared 8×8-inch pan. Bake for 20–25 minutes, until the center appears set and a toothpick inserted in the center comes out clean. Remove from the oven and let the brownies cool completely in the pan.

Prepare the mint layer: in a medium bowl, beat 3 tablespoons unsalted butter (softened) together with 1 ½ cups powdered sugar until smooth and combined.

Add 1 ½ tablespoons milk and ¾ teaspoon pure peppermint extract to the butter–sugar mixture. Beat until smooth. Add 2 drops green food coloring and mix until the color is uniform.

Spread the mint filling evenly over the cooled brownie base. Refrigerate the pan until the mint layer is firm (chilled), about until it holds its shape when touched.

Prepare the chocolate topping: melt 1 ½ ounces unsweetened chocolate (1 ½ 1‑ounce squares) and 1 ½ tablespoons unsalted butter together in a heatproof bowl over simmering water, stirring until smooth. (You may also melt gently in short bursts in the microwave, stirring between bursts.)

Drizzle the melted chocolate over the chilled mint layer, then use a spatula to spread it gently into a thin, even layer covering the surface.

Return the pan to the refrigerator and chill until the topping is set. Cut into squares while chilled—the brownies will cut more cleanly when cold.