Prep the pan. Line an 8-inch square pan with parchment, leaving an overhang on two sides for easy lifting. Preheat the oven to 350°F (175°C).

Melt butter and chocolate. In a heatproof bowl, melt the butter and chopped chocolate together over a double boiler or in 20–30 second microwave bursts, stirring until smooth. Let it cool for 3–4 minutes so it’s warm, not hot.

Whisk in sugars. Add granulated and brown sugars to the melted chocolate mixture. Whisk until glossy and combined.

Add eggs and vanilla. Whisk in the eggs, one at a time, until the batter thickens and looks shiny. Stir in vanilla.

Fold in dry ingredients. Sift in the flour, cocoa, and salt. Fold gently with a spatula just until you see no dry streaks. Don’t overmix.

Bake the brownie base. Spread the batter evenly in the prepared pan. Bake for 22–26 minutes, until the edges are set and a toothpick comes out with a few moist crumbs. Do not overbake.

Cool completely. Let the brownies cool in the pan on a rack. You can speed this up by placing the pan in the fridge for 20–30 minutes once it’s no longer hot.

Make the mint layer. Beat the softened butter until creamy. Add powdered sugar, peppermint extract, and a pinch of salt. Beat on low, then increase to medium, adding 2–3 tablespoons of cream as needed until spreadable and fluffy. Tint light green if you like.

Spread the mint layer. Once the brownie base is cool, spread the mint cream evenly over the top. Chill for 20–30 minutes so the layer firms up.

Prepare the glaze. Melt chocolate and butter together until smooth. Stir in 1–2 teaspoons corn syrup or honey if you want extra shine and a slightly softer set.

Glaze and set. Pour the glaze over the chilled mint layer. Tilt the pan to coat evenly or smooth with an offset spatula. Chill again for at least 30–45 minutes, or until the glaze is set.

Slice cleanly. Lift the slab out using the parchment. Warm a sharp knife under hot water, wipe it dry, and cut into squares. Wipe and rewarm the knife between cuts for neat edges.