Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides.
Melt the butter and chopped chocolate together in a large microwave-safe bowl in 30 second bursts, stirring each time. Whisk in both sugars, cool for 5 minutes, then whisk in the sour cream and vanilla until glossy.
In a separate bowl, whisk the flour, cocoa powder, baking soda and salt. Fold into the chocolate mixture just until combined and spread evenly into the pan.
Bake for 25 to 30 minutes, until a toothpick inserted in the center comes out with moist crumbs. Let cool completely in the pan, about 2 hours.
For the mint layer, beat the softened butter until creamy, then add the powdered sugar, milk, peppermint extract and green food coloring. Beat for 2 to 3 minutes until fluffy.Spread it evenly over the cooled brownies and refrigerate for 30 minutes, until the mint layer is firm to the touch.
For the topping, heat the heavy cream until steaming, pour it over the chocolate chips and let it sit for 5 minutes. Stir until smooth, then let it cool for 10 minutes so it is lukewarm and will not melt the mint layer.
Pour the chocolate over the chilled mint layer and gently spread it to the edges. Refrigerate for 30 to 45 minutes until set, then cut into 16 squares.