Preheat the oven to 350°F. Line a 9x13 inch baking pan with parchment paper, leaving plenty of overhang on the long sides, and lightly spray with non-stick cooking spray.

In a large mixing bowl, combine the butter and 3 oz of the chopped chocolate. Melt in the microwave in 15–30 second bursts, stirring well each time, until smooth and fully melted.

Add the granulated sugar and beat with a handheld electric mixer until combined. The mixture will look grainy at this stage which is normal and expected.

Add the eggs and vanilla extract, then beat on high speed for about 1 minute, until fully combined, glossy, and slightly thickened. This step helps create structure.

In a separate bowl, whisk together the flour, cocoa powder, and salt. Add the dry ingredients to the wet ingredients and beat on a low speed just until no dry patches of flour remain.

Fold in the remaining 3 oz chopped chocolate. The batter will be thick.

Spread evenly into the prepared pan, smoothing the top with an offset spatula or the back of a spoon.

Bake in the center of the oven for 28–32 minutes, checking at 28 minutes. The edges should be set and a toothpick inserted in the center should come out with moist crumbs, not wet batter. Do not over bake, the brownies should be sturdy but still fudgy.

Remove from the oven and allow to cool in the pan for 15–20 minutes before adding the mint layer. The brownies should be warm or fully cool, but not hot.

In a medium bowl, beat the softened butter until completely smooth and creamy.

Gently spread the mint filling evenly over the brownie base so the brownie layer isn’t disturbed.

Place the pan in the refrigerator and chill for 45–60 minutes, until the mint layer is fully firm to the touch.

Combine the butter and chocolate chips in a saucepan over medium heat, stirring constantly until melted and smooth. Once smooth, let the ganache sit for about 5 minutes. It will still be quite runny, which is normal and gives a perfectly smooth, mirror-like finish.

Pour the ganache over the chilled mint layer, then tilt the pan in all directions to spread it into an even layer.

Refrigerate uncovered for 45–60 minutes, until the surface is set and no longer tacky. Once set, the pan can be loosely covered and chilled for another 1–2 hours until fully firm.

Lift the brownies out using the parchment overhang. Let them sit at room temperature for 10–15 minutes before slicing. Use a large knife warmed under hot water, wipe dry between cuts, and press straight down (no sawing) for clean layers and a smooth ganache top.