How to Make It
Preheat the oven to 350° and line a 9-inch-square baking pan with foil, allowing 2 inches of overhang on 2 sides. In a medium glass bowl, microwave the chocolate on high power in 20-second intervals until the chocolate is melted; stir between intervals. Let cool slightly.
In a medium bowl, whisk the flour with the cocoa powder, baking powder and baking soda. In another medium bowl, whisk the buttermilk with the coffee and vanilla.
In a stand mixer fitted with the paddle or with a handheld electric mixer, beat the butter with the sugar and salt at medium speed until fluffy, about 2 minutes. Beat in the eggs 1 at a time until incorporated. At low speed, in 3 alternating batches, beat in the dry ingredients and the buttermilk mixture, scraping down the side of the bowl as necessary. Fold in the melted chocolate.
Scrape the batter into the prepared pan. Bake for about 40 minutes, until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 45 minutes. Using the foil, transfer the cake to a rack to cool completely.
In a glass bowl, microwave the chocolate on high power in 20-second intervals until melted; stir between intervals. Let cool slightly. Scrape the chocolate into the bowl of a stand mixer fitted with the whisk. At medium speed, beat in the cream and confectioners' sugar until smooth. At high speed, gradually add the butter and beat until the frosting is thick.
Transfer the chocolate cake to a serving platter and discard the foil. Spread the buttercream frosting evenly over the top and sprinkle with cocoa nibs. Cut the cake into squares and serve.