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Preheat the oven to 325F/160C.
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Line the bottom of a 8-inch round cake pan with parchment, and either butter and lightly flour the parchment and exposed sides of the pan, or spray the interior with a nonstick spray.
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In a large bowl, on the medium speed of an electric mixer, cream the butter until smooth. Add the brown sugar and ¼ cup of the white sugar, and beat until fluffy, about 3 minutes.
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Add the egg and yolk and beat well, then add the vanilla and ¾ cup of the xinomavro wine (reserving the rest for the sauce).
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Sift the flour, cocoa, baking soda, baking powder, cinnamon and salt together, directly into the wet ingredients.
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Mix until almost combined, then fold the rest together with a rubber spatula.
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Spread batter in prepared pan. Bake for 30 to 35 minutes, or until a cake tester inserted into the center comes out clean. The top of the cake should be shiny and smooth. Remove the cake and let it cool on a rack.
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While the cake is cooling, prepare the syrup and the garnish: For the syrup, cook the remaining cup of wine and ¾ cup of white sugar together with the cinnamon stick, peppercorns and cloves in a small saucepan over medium heat until the mixture is reduced by about half and reaches the consistency of a thick syrup. Strain it and discard the cinnamon, peppercorns and cloves.
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For the garnish, when the cake is done, raise the oven temperature to 375F / 190C. Place the grape clusters on a shallow baking pan lined with parchment or baking paper and roast for about 20 minutes, turning, until wrinkled and pleasantly “leathery.” Remove and set aside.
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To serve: Cut the cake into serving pieces and top each piece with a few individual roasted grapes or a small cluster of roasted grapes. Drizzle with the syrup, and spoon a little whipped cream, if desired, onto the plate or cake. Sift a little cocoa onto the plate or grind a little black pepper over and around the plate and serve.