Preheat the oven to 350 degrees F. Grease a 9 by 13-inch baking pan.
In a large mixing bowl, sift together the flour, baking powder, and salt. Reserve.
In a double boiler, combine the butter and chocolate. Melt over medium heat. When melted, remove top portion of double boiler. Set aside and allow to cool for about 10 minutes, stirring once or twice. Whisk in the sugar, eggs, and vanilla extract and mix until fully combined. Stir in the sour cream. Fold in the flour mixture until just combined. Do not overmix.
Pour the batter into the prepared pan and spread evenly. Bake for 25 to 30 minutes. Allow the brownie to cool on a wire rack for about 45 minutes before continuing.
To make the mint cream, melt the butter in a medium saucepan over medium heat. Add the peppermint extract, cream, and confectioner's sugar; whisk until well combined. Cook the mixture for about a minute over medium heat. Spread evenly over the brownies. Allow the mint cream to set for about 10 minutes before continuing.
For the ganache, melt the butter and chocolate in a double boiler over medium heat. Add cream and stir until well combined and smooth. Spread the ganache evenly on top of the mint cream layer and top with crushed chocolate mints if desired. Allow to set until very firm, about 2 hours in the refrigerator.