Spray a 9x4 1/2 x3" 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 simming water until sugar dissolves and egg whites are warm ( but not hot ), about 5 minutes. Remove from heat. Usingan electric mixer, beat on medium-high speed until cool, tripled in volume., and stiff with peaks form, about 8 minutes. ( if egg whites look grainy, you've gone to 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 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.
Bring first 5 ingredients to a boil in a small saucepan. Remove from heat and cover. Let steep for 15 minutes.
Place chocolate in a medium heatproof bowl; set aside. Strain milk mixture through a fine-mesh sieve into a medium heatproof bowl; return to saucepan. Bring to a boil. Gradually pour milk mixture over chocolate, stirring until melted and smooth. Stir in tequila, if using. DO AHEAD; Sauce can be made 2 days ahead. Let cool completely. Cover and chill.
gently rewarm befor continuing.
Cinnamon Whipped cream and assembly;
Combine cream,sugar,and cinnomon in a medium bowl; scrape in seeds from vanilla bean; discard bean. Using an electric mixer,beat until soft peaks form.
Uncover semifreddo and invert onto a platter. Lift pan and discard plastic wrap. Using a knife dipped in hot water and wiped dry, cut semifreddo into clean slices. Transfer to plates. Garnish with cinnamon whipped cream and warm chile-chocolate sauce, if desired. Sprinkle almonds over.
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