This recipe is adapted from Michele Goldsmith, Executive Pastry Chef at Nobu 57. Martha made this on episode 506 of Martha Bakes.
Trim a sheet of acetate to 34-by-3-inches and line the walls of a 10-inch springform pan; set aside.
Place 2 ounces white chocolate in the top of a double boiler or heatproof bowl set over a saucepan of simmering water; heat just until chocolate melts. Stir in tahini, then stir in cereal, 1/4 cup desiccated coconut, 1/3 lemon zest and salt.
Place cake, brushed-side down, on a wire rack set over a rimmed baking sheet. Spread cereal-mixture over surface of cake in an even layer. Freeze until set, about 1 hour.
Carefully invert cake into the prepared springform pan so crunch-side is down. Using an offset spatula, spread cake with 1 cup roasted pineapple puree in an even layer; reserve remaining puree for topping, if desired. Freeze until set, about 1 hour.
Meanwhile, place gelatin in a small amount of cold water until softened, about 1 minute. Drain and squeeze to remove excess water from gelatin; set aside.
In a small saucepan, bring coconut milk, remaining 1/2 cup desiccated coconut and lemon zest to a boil over medium-high heat. Remove from heat. Add reserved gelatin, and whisk until dissolved. Strain mixture through a fine sieve set over a bowl with remaining 3 1/4 ounces white chocolate; let stand until chocolate begins to melt. Using a rubber spatula, gently stir from the center of the bowl outward to avoid forming air bubbles. Let cool at room temperature, stirring occasionally, to prevent mixture from setting up (mixture should be thick, but not clumpy).
Meanwhile, whisk cream until soft peaks form. Place in refrigerator to chill.
In a mixing bowl, whisk egg whites on medium speed until frothy. Gradually add sugar, and increase speed to high, beating until soft peaks form. Reduce speed to low and gradually add about 1/3 of gelatin mixture, whisking until just combined.
Gently fold half of egg-white mixture into remaining gelatin mixture. Fold half of the chilled whipped cream into the gelatin mixture. Fold in remaining egg-white mixture, then fold in remaining whipped cream until mousse is thoroughly combined.
Pour mousse over the cake. Freeze until set, at least 8 hours or up to overnight.
Once mousse sets, remove ring from springform pan and carefully peel off acetate. If desired, top with coconut flakes, reserved chopped roasted pineapple, and reserved pineapple puree. Return to freezer. Transfer to refrigerator two hours before serving.
Silver-strength gelatin is a gelatin sheet commonly used in professional kitchens. The "silver" refers to the grade of bloom-strength, or the firmness of the gel once it sets.