Chocolate Semifreddo with Chile-Chocolate Sauce

Spray a 9x4 1/2 x 3" loaf pan with nonstick spray. Line with plastic wrap, leaving a 2" overhang; set aside. Whisk egg whites and sugar in a medium heatproof bowl set over a medium saucepan of simmering water until sugar dissolves and egg whites are warm (but not hot), about 5 minutes. Remove from heat. Using an electric mixer, beat on medium-high speed until cool, tripled in volume, and stiff peaks form, about 8 minutes. (If egg whites look grainy, you've gone too far.) Stir chocolate and butter in a large heatproof bowl over same saucepan of simmering water until melted and smooth; remove from heat; let cool slightly. Beat cream in another medium bowl until medium-stiff peaks form (cream should be soft and pillowy; peaks will just fall onto themselves). Using a rubber spatula, gently fold egg whites into warm chocolate. Fold in whipped cream. (Stop folding as soon as mixture becomes one smooth texture and light brown color.) Scrape semifreddo into prepared pan; smooth top. Cover tightly and freeze until firm, 3-4 hours. DO AHEAD: Semifreddo can be made 5 days ahead. Keep tightly wrapped and frozen.