Preheat the oven to 350 degrees F. Grease and flour 6 individual serving baking molds, such as the flexible non stick 100 percent silicone molds.
Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment for about 5 minutes, or until light and fluffy. Add the eggs, 1 at a time, then the orange zest.
Sift together 1 1/2 cups flour, the baking powder, baking soda, and salt in a large bowl. In another bowl, combine the orange juice, buttermilk, and vanilla. Add the flour and buttermilk mixtures alternately in thirds to the creamed butter, beginning and ending with the flour. Toss the chocolate chunks with 1 tablespoon flour and add to the batter. Pour into the pans, smooth the tops, and bake for 30 minutes, until a cake tester comes out clean. Let the cakes cool in the molds on a wire rack for 10 minutes.
Meanwhile, make the syrup. In a small saucepan over medium-low heat, cook the sugar with the orange juice until the sugar dissolves. Remove the cake from the pans, put them on a rack over a tray, and spoon the orange syrup over the cakes. Allow the cakes to cool completely.
For the ganache, melt the chocolate, heavy cream, and coffee in the top of a double boiler over simmering water until smooth and warm, stirring occasionally. Drizzle over the top of the cakes.
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