Equipment: One 8x2 1/2 or higher springform pan, greased and bottom lined with greased parchment or wax paper; outside of the pan wrapped in a double layer of aluminum foil to prevent seepage.

Cake: Preheat oven to 350 degrees. In a large mixing bowl, beat the cream cheese and sugar until very smooth (about three minutes), preferably with a whisk beater. Add the eggs, one at a time, beating after each addition until smooth, scraping down the sides. Add lemon juice, vanilla, and salt and beat until incorporated. Beat in the sour cream just until blended. Cool melted chocolate and blend into batter after the sour cream is incorporated. Pour the batter into the prepared pan. Set the pan in a larger pan and surround it with one inch of very hot water. Bake 45 minutes. Turn oven off without opening door and let cake cool in the oven for 1 hour. Remove to a rack and cool to room temperature (about 1 hour). Cover with plastic wrap and refrigerate overnight. Have ready a serving plate and a flat plate at least 8 inches in diameter, covered with plastic wrap. Place the springform pan on a heated burner and move it around for 15 seconds. Wipe sides of pan with a hot, damp towel. Run a thin metal spatula around the sides of the cake, and release the springform pan. Place the plastic-wrapped plate on top and invert. Remove the bottom of the pan and the parchment. Reinvert onto the serving plate and use a small spatula to smooth the sides. Frost and refrigerate until shortly before serving. Frosting: Break chocolate into squares and place in top of a double boiler over hot water (but not hotter than 160 degrees) on low heat. The water must not touch the bottom of the double boiler insert. Remove the double boiler from heat and stir until the chocolate begins to melt. Return to heat if the water cools, but be careful not to let it get too hot. Stir 10 minutes or until smooth. (The chocolate may be melted in a microwave oven if stirred every 15 seconds.) Remove before completely melted and stir. The chocolate will continue melting using residual heat. Allow to cool. In a mixing bowl, beat cream cheese (preferably with a flat beater) until smooth and creamy. Gradually beat in the cooled chocolate until smooth. Beat in the butter and lemon juice. Before frosting the cake, allow frosting to reach room temperature and beat again to ensure smoothness