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1
Make the Cake Preheat the oven to 350 and line a 9-inch-square baking pan with foil, allowing 2 inches of overhang on 2 sides.
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2
In a medium glass bowl, microwave the chocolate on high power in 20-second intervals until the chocolate is melted; stir between intervals.
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3
Let cool slightly.
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4
Make the Cake In a medium bowl, whisk the flour with the cocoa powder, baking powder and baking soda.
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5
In another medium bowl, whisk the buttermilk with the coffee and vanilla.
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6
Make the Cake 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.
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7
Beat in the eggs 1 at a time until incorporated.
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8
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.
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9
Fold in the melted chocolate.
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10
Make the Cake Scrape the batter into the prepared pan.
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11
Bake for about 40 minutes, until a toothpick inserted in the center comes out clean.
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12
Let the cake cool in the pan for 45 minutes.
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13
Using the foil, transfer the cake to a rack to cool completely.
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14
Meanwhile, Make the Buttercream In a glass bowl, microwave the chocolate on high power in 20-second intervals until melted; stir between intervals.
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15
Let cool slightly.
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16
Scrape the chocolate into the bowl of a stand mixer fitted with the whisk.
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17
At medium speed, beat in the cream and confectioners' sugar until smooth.
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18
At high speed, gradually add the butter and beat until the frosting is thick.
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19
Meanwhile, Make the Buttercream Transfer the chocolate cake to a serving platter and discard the foil.
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20
Spread the buttercream frosting evenly over the top and sprinkle with cocoa nibs.
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21
Cut the cake into squares and serve.
