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1
Lightly butter an 8-inch springform cake pan and line the base with a round of baking parchment. Heat the oven to 350u00b0 F.
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2
Cook the beets, whole and unpeeled, in boiling unsalted water. Depending on their size, they will be tender within 30 to 40 minutes. Young ones may take slightly less. Drain them, let them cool under running water, then peel them, slice off their stem and root, and process in a blender or food processor until a coarse puree.
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3
Melt the chocolate, broken into small pieces, in a small bowl set over a pot of simmering water. Don't stir.
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4
When the chocolate looks almost melted, pour the hot espresso over it and stir once. Cut the butter into small pieces-the smaller the better-and add to the melted chocolate. Push the butter down under the surface of the chocolate with a spoon (as best you can) and leave to soften.
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5
Sift together the flour, baking powder and cocoa. Separate the eggs, putting the whites in a large mixing bowl. Stir the yolks together.
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6
Now, working quickly but gently, remove the bowl of chocolate from the heat and stir until the butter has melted into the chocolate. Let sit for a few minutes, then stir in the egg yolks. Do this quickly, mixing firmly and evenly so the eggs blend into the mixture. Fold in the beets. Whisk the egg whites until stiff, then fold in the sugar. Firmly but gently, fold the beaten egg whites and sugar into the chocolate mixture. A large metal spoon is what you want here; work in a deep, figure-eight movement but take care not to over-mix. Lastly, fold in the flour and cocoa.
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7
Transfer quickly to the prepared cake pan and put in the oven, decreasing the hea immediately to 325u00b0 F. Bake for 40 minutes. The rim of the cake will feel spongy, the inner part should still wobble a little when gently shaken. Test with a cake tester or toothpick too-if it is still gooey in the center, continue baking just until moist crumbs cling to the tester.
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8
Set the cake aside to cool (it will sink a tad in the center), loosening it around the edges with a thin icing spatula after half an hour or so. It is not a good idea to remove the cake from its pan until it is completely cold. Serve in thick slices, with creme fraiche and poppy seeds.
