Brownies:

Preheat oven to 330° F (165° C). Line an 8×8-inch square baking tin with parchment in a criss-cross-pattern, allowing a 1-inch overhang on all sides. Spritz lightly with nonstick spray. Set aside.

Melt butter in a saucepan over medium heat. Whisk in granulated sugar, cocoa powder, and kosher salt. Remove from heat and allow to cool slightly, about 5 minutes. (Mixture will be thick.)

Transfer to a large mixing bowl. Whisk in eggs, vanilla extract, and peppermint oil.

With a flexible rubber spatula or wooden spoon, fold in the flour until just combined. Do not overmix.

Fold in half of the chopped Andes mints (or mint chocolate) to the batter, reserving remaining chocolate for decoration.

Spread into prepared tin.

Bake in preheated oven for 35-40 minutes, or until a toothpick or cake tester comes out mostly clean. Immediately distribute chopped dark chocolate evenly over the top. Wait 1-2 minutes, allowing the chocolate to melt slightly. (If necessary, return to warm oven for 30 seconds to melt the chocolate.) With a small stepped palette knife or rubber spatula, spread the melted chocolate, completely covering the brownie slab.

Transfer to a wire rack. Cool, uncovered, at room temperature for 1 hour, or until chocolate sets and bottom of pan feels completely cool to the touch.

Mint Buttercream:

In a large 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 1-2 tablespoons of milk. If frosting seems too thick to spread, add a splash more milk. If too thin, add more confectioner’s sugar.

Mix in peppermint oil and mint green gel 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.

With a sharp chef’s knife, slice into squares and serve.

Store leftover brownies in an airtight container in the refrigerator for up to 5 days.