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1. Position a rack in the center of the oven and preheat to 350u00b0F. Butter and flour the inside of a 8 to 10-cup fluted tube pan and tap out the excess flour.
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2. Combine the milk and lemon juice in a glass measuring cup. Let stand in a warm place (near the preheating oven) while preparing the rest of the batter; the milk will curdle. Sift the flour, cocoa, baking soda, and salt together into a medium bowl.
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3. Bring 1/2 inch of water to a simmer in a small saucepan, and turn off the heat. Place the chocolate in a custard cup or ramekin and set in the hot water. Let stand until the chocolate is melted, then remove from the water, being careful not to splash any water into the chocolate, and stir until smooth. Let stand until tepid.
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4. Beat the butter in the bowl of a heavy-duty stand mixer fitted with the paddle attachment on medium-high speed until smooth, about 1 minute. Gradually beat in the sugar, then add the vanilla. Beat until the mixture is very light in color and texture, scraping occasionally, about 4 minutes. Gradually beat in the eggs. Reduce the mixer speed to low. Beat in the cooled chocolate. In thirds, alternating with two equal additions of the milk mixture, add the flour mixture, scraping down the bowl and beating until smooth after each addition. Spoon the batter into the pan and smooth the top with a spatula.
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5. Bake until the top of the cake springs back when gently pressed with your finger, and a cake tester inserted into the center of the cake comes out clean, about 1 hour. Cool on a wire rack for 10 minutes. Invert and unmold the cake onto the rack and cool completely. Sift a light coating of confectioners' sugar on top, if using. (The cake can be stored at room temperature, wrapped in plastic wrap, for up to 2 days.)
