Brownies:

Preheat oven to 330° F / 165° C. Line an 8 x 8-inch / 20 x 20-centimeter square baking tin with parchment, grease lightly with butter or cooking spray, and set aside.

Melt butter in a saucepan over medium heat. When butter has melted, whisk in sugar, cocoa powder, and salt. Remove from heat and let cool slightly, about five minutes. Mixture will be thick.

Transfer butter-sugar-cocoa mixture to a large mixing bowl.

Add eggs, vanilla, and peppermint oil to butter-sugar-cocoa mixture, and whisk to combine.

With a wooden spoon, fold in the flour until just combined.

Add half of the chopped Andes mints (or mint chocolate) to the batter, stirring until fully incorporated. Spread batter into prepared baking tin.

Bake in preheated oven for 35 – 40 minutes, or until a toothpick or cake tester comes out mostly clean, with just a few crumbs visible.

Remove tin from oven, and immediately distribute chopped dark chocolate evenly over the top. Wait a minute or two, allowing the chocolate to melt slightly. (If necessary, you can return the tin to the warm oven for 30 seconds to melt the chocolate.) With a rubber spatula, spread the melted chocolate over the top of the brownies, covering completely.

Transfer baking tin to a wire rack and allow brownies to cool completely, at least one hour.

When brownies have cooled and chocolate topping has set, prepare the mint buttercream.

Mint Buttercream:

In a mixing bowl or the bowl of a stand mixer, whip butter on high speed until light and fluffy.

Add the confectioner’s sugar, 1/2 cup at a time, mixing well after each addition.

Mix in in 1-2 tablespoons of milk, adding just enough to smooth the frosting, but not enough to make it too thin. If mixture still seems to thick, add a tiny bit more liquid. If too thin, add more confectioner’s sugar.

Mix in the peppermint oil and the green food coloring.

Assembly:

With a rubber spatula or small palette knife, spread a thick layer of buttercream over the chocolate layer.

Sprinkle with remaining mint chocolate pieces. Cut into squares.

Store leftover brownies in an airtight container in the refrigerator for up to three days.