Step 1

Arrange a rack in the middle of the oven and heat to 350°F. Line an 8x8-inch baking pan with parchment paper so that it hangs over two sides of the pan. Lightly grease the pan and parchment with cooking spray or butter.

Step 2

Melt the butter and chocolate in a medium saucepan over low heat, stirring constantly. Remove the pan from the heat and whisk in the cocoa powder, sugar, and vanilla until no lumps remain. Let sit for 5 minutes so the cocoa powder can bloom. Whisk the eggs in one at a time, mixing well after each addition. Add the flour and salt and fold in with a rubber spatula.

Step 3

Transfer to the prepared baking pan and spread in an even layer. Bake until set, 23 to 25 minutes. Cool completely in the pan.

Step 4

Place the butter and cream cheese in a stand mixer fitted with the paddle attachment. (Alternatively, use an electric hand mixer and large bowl.) Beat on medium speed until smooth, about 2 minutes. Beat in the crème de menthe, peppermint extract, and salt.

Step 5

With the mixer on low speed, gradually add in the sugar. Once incorporated, increase the speed to medium-low and mix until smooth and fluffy.

Step 6

Spread in an even layer over the brownies. Refrigerate at least 30 minutes. Meanwhile, make the ganache.

Step 7

Place the chocolate in a medium heatproof bowl. Heat the cream in a small saucepan over low heat until bubbling round the edges, then pour it over the chocolate mixture. Let sit for about 10 minutes, then stir with a spatula until completely smooth. Let it cool at room temperature for about 10 minutes, then use it as soon as it starts to thicken up.

Step 8

Remove the brownies from the refrigerator and spread the ganache in an even layer over the buttercream with an offset spatula or knife. Refrigerate for at least 30 minutes before slicing and serving.