For the brownies: Preheat the oven to 350 degrees F. Grease an 8-inch square baking pan with butter and line with parchment paper.
Place the butter and chocolate in a medium, non-reactive metal bowl and set over simmering water. Stir occasionally and whisk in the cocoa powder, chile powder, cinnamon and chipotle powder until smooth, then set aside.
In a mixing bowl, whisk together the sugar, vanilla, salt and eggs for about 30 seconds. Add the warm chocolate mixture to the sugar mixture and then mix in the flour just until combined, to avoid over-mixing. Pour the batter into the prepared pan and cook for 35 minutes. Cool the brownies for at least 1 hour before frosting.
For the frosting: In the bowl of a stand mixer fitted with the paddle attachment, combine the confectioners' sugar, cocoa powder, butter, boiling water, salt and vanilla extract. Beat on low speed for 1 minute, then beat on high speed for 2 minutes.
Frost the brownies, then sprinkle some caramelized cacao nibs on top to garnish. Cut into 1-inch squares and serve.