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1
Position a rack in the middle of your oven and preheat to 350u00b0F/ 180u00b0C. Grease a 9-by-5-in/23-by-12-cm loaf pan.
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2
Sift together the pastry flour, all-purpose flour, cocoa powder, baking powder, and baking soda. Set aside.
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3
Combine the coffee, buttermilk, and vanilla. Set aside. Melt 3/4 cup/ 130 g of the chocolate gently over a double boiler or in a small bowl set over a small saucepan of simmering water. Remove the pan from the heat, but leave the chocolate over the double boiler to keep warm while you mix the cake. In a stand mixer fitted with the paddle attachment, cream the butter, sugar, and salt on medium-high speed until light and fluffy, about 2 minutes. Incorporate the eggs, one at a time, beating well after each addition. Be sure to scrape the sides of the bowl well. Pause mixing and add the flour mixture. Mix just until incorporated. With the mixer on low speed, pour in the coffee mixture. Fold in both the melted and chopped chocolate.
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4
Pour the batter into the prepared pan and bake for 55 minutes, or until a cake tester comes out clean. Do not overbake! Allow to cool completely.
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5
Place a flat plate on top of the cake and pan. Carefully invert the cake onto the plate by flipping both upside down. Then lift the pan off the cake. Rest your serving plate on the bottom of the cake and turn the cake right-side up onto the plate. Once cooled, top with powdered sugar, if desired.
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6
This keeps beautifully, tightly wrapped, at room temperature, for up to 4 days.
