Instructions

for the fudge sauce

 Combine 1 cup sugar, butter, and milk in small saucepan. Bring to gentle simmer over medium-low heat and simmer, stirring occasionally, for 3 minutes. Meanwhile, stir cocoa, cream, salt, and remaining ¼ cup sugar in bowl until no lumps of cocoa remain. Reduce heat to low and whisk cocoa mixture into sugar-butter mixture in saucepan. Cook, stirring occasionally, until sugar dissolves, about 5 minutes.
 Off heat, add chocolate and stir until well combined. Stir in vanilla. Transfer sauce to bowl and let cool completely, about 1¼ hours. (Sauce can be refrigerated for up to 1 week. Let sit at room temperature for at least 1 hour before using.)

for the chiffon

 Adjust oven rack to middle position and heat oven to 300 degrees. Lightly grease bottom only of 9-inch springform pan and line bottom with parchment paper. Secure collar to bottom of pan. Whisk flour, 6 tablespoons sugar, baking powder, and salt in bowl until well combined. Add whole egg and yolks, oil, water, and vanilla and whisk until smooth batter forms.
 Using hand mixer, whip egg whites and cream of tartar on medium-low speed in separate bowl until foamy, 30 to 60 seconds. Sprinkle remaining 2 tablespoons sugar over whites. Increase speed to medium and whip whites until glossy, stiff peaks form, 1 to 2 minutes. Transfer one-third of whites to batter; gently whisk until few streaks of white remain. Using rubber spatula, fold remaining whites into batter.
 Transfer batter to prepared pan and bake until toothpick inserted in center comes out clean, 20 to 25 minutes. Transfer to wire rack and let chiffon cool completely in pan, about 45 minutes. Run thin knife between chiffon and side of pan; remove side of pan. Invert chiffon onto wire rack and remove parchment. Reinvert chiffon. (Cooled chiffon can be stored in airtight container at room temperature for up to 12 hours or frozen for up to 1 month.)

for the cake

 Twenty minutes before assembling cake, place ice cream in refrigerator to soften. While ice cream softens, prepare your work station. Lightly grease 9-inch springform pan; line bottom with parchment; line sides with overlapping 3-inch-wide strips of parchment; and place pan in freezer. Measure out ¾ cup fudge sauce and place large spoon, clean dish towel, 12-inch length of plastic wrap, and chiffon nearby.
 Working quickly, use large spoon to transfer ice cream to prepared pan. Place sheet of plastic directly over ice cream. Cover plastic with dish towel. Press firmly with your hands to flatten ice cream into even layer. Remove dish towel and plastic. Spread reserved fudge sauce evenly over ice cream. Invert chiffon on top of fudge and gently press chiffon into fudge. Wrap tightly with plastic and freeze for at least 4 hours or up to 1 week.
 Freeze serving platter for at least 20 minutes. Whip cream, sugar, and vanilla in bowl until stiff peaks form, 3 to 5 minutes. Transfer two-thirds of cream to pastry bag fitted with ½-inch flat petal tip or ½-inch round tip and refrigerate remaining cream. Unwrap cake and remove side of pan. Remove parchment strips. Invert serving platter on top of cake. Invert cake onto platter; remove bottom of pan and parchment.
 Working quickly, pipe whipped cream evenly onto sides and top of cake. (It's OK if there are small gaps between bands of whipped cream.) Using offset spatula, smooth whipped cream into even layer. Freeze cake until whipped cream at bottom edge of cake is firm, about 20 minutes (bottom edge will be last place to harden). Transfer remaining whipped cream to pastry bag. Pipe decorative border along top edge of cake. Freeze until cake is completely hardened, at least 1 hour or up to 24 hours.TO SERVE: Thirty minutes before serving, transfer cake to refrigerator. Microwave remaining fudge sauce at 50 percent power until fluid, about 1 minute, stirring halfway through microwaving. Slice cake, wiping blade between cuts, and serve, passing remaining fudge sauce separately.