Preheat oven to 350°F. Oil a 15- by 10- by 1-inch baking pan and line bottom lengthwise with a large piece of wax paper, allowing a 2-inch overhang on each end. Stir together espresso powder and hot water in a heavy saucepan until coffee is dissolved. Add chocolate and melt over low heat, stirring, until smooth. Remove from heat and cool to room temperature. Beat together yolks, 1/3 cup sugar, and 1/8 teaspoon salt in a large bowl with a handheld electric mixer at moderately high speed until thick and pale, 5 to 7 minutes. Beat in melted chocolate. Beat whites with remaining 1/8 teaspoon salt in another large bowl with cleaned beaters until they just hold soft peaks. Gradually add remaining 1/3 cup sugar and beat until whites just hold stiff peaks. Stir one third of whites into chocolate mixture to lighten, then fold in remaining whites gently but thoroughly. Spread batter evenly in baking pan and bake in middle of oven until puffed and top is dry to the touch and springs back when gently pressed, 12 to 14 minutes. Cover cake with 2 layers of dampened paper towels and let stand in pan on a rack 3 minutes, then remove towels and cool completely. Loosen edges with a sharp knife. Sift cocoa powder evenly over top of cake and overlap 2 layers of wax paper lengthwise over cake. Invert a baking sheet over cake, then invert cake onto it, gently peeling off wax paper now on top. Lightly oil loaf pan and line with 2 (24-inch-long) crisscrossed sheets of plastic wrap, letting excess hang over all sides. Using outside of loaf pan as a stencil, cut a rectangle from cake to line bottom of pan. Cut another rectangle for top of cake. Cut 2 pieces of cake to line long sides of pan, then 2 more for short sides. Fit all cake pieces (except top piece) into pan, cocoa sides against pan, pressing gently to help adhere. Wrap top piece of cake in plastic wrap and cover cake in pan with plastic-wrap overhang, then freeze cake while making semifreddo. Beat together eggs and brown sugar in a metal bowl set over a saucepan of simmering water with electric mixer at medium speed until mixture registers 160°F on an instant-read thermometer, 6 to 8 minutes. Remove bowl from heat and chill mixture until cool, about 10 minutes. Mix together cream, vanilla, and malted milk powder in a separate bowl at low speed with electric mixer until powder is dissolved, then increase speed to moderately high and beat until it just holds soft peaks. Stir one third of cream into egg mixture to lighten, then fold in remaining cream gently but thoroughly. Spoon semifreddo into cake-lined pan, spreading evenly and smoothing top, and cover with top piece of cake. Freeze, covered with plastic-wrap overhang, until firm, at least 8 hours. Before serving, let cake stand at room temperature 5 minutes. Unwrap plastic and invert cake onto a long platter, using plastic wrap to help pull cake from pan. Sift cocoa evenly over top to garnish, then top with chocolate curls and cut into 1/2-inch-thick slices. Serve immediately.