For praline and chocolate curls: Place sheet of foil on work surface. Bring sugar and 1/4 cup water to boil in heavy medium saucepan over medium-high heat, stirring until sugar dissolves. Boil without stirring until syrup is deep amber, brushing down sides of pan with wet pastry brush and swirling occasionally, 7 to 8 minutes. Mix in espresso powder, then nuts. Scrape mixture onto foil; quickly spread out. Cool completely. Chop praline into small pieces.
Microwave white chocolate block at 50 percent power in 10-second intervals until chocolate feels just slightly warm and is just beginning to soften (if too soft, let stand until firm enough to shave). Using vegetable peeler, shave 1- to 1 1/2-inch-wide ribbon, about 4 to 5 inches long, from block. Gently roll ribbon into loose curl (cylinder). Repeat, making 20 to 24 curls. DO AHEAD: Praline and chocolate curls can be made 2 days ahead. Chill separately in airtight containers.
For mousse: Stir chocolate and 5 tablespoons water in metal bowl set over saucepan of barely simmering water until chocolate is smooth. Remove bowl from over water. Cool chocolate to lukewarm, stirring occasionally. Using electric mixer, beat cream in large bowl until soft peaks form. Gradually pour lukewarm chocolate over cream, folding to blend well. Transfer 2 1/2 cups mousse to another medium bowl; gently fold in espresso powder. Cover and chill both bowls of mousse until firm, at least 6 hours. DO AHEAD: Can be made 1 day ahead. Keep chilled.
For soaking syrup and ganache: Stir liqueur and espresso powder in small bowl until espresso dissolves. Place chocolate and corn syrup in medium bowl. Bring cream to boil in small saucepan; pour over chocolate and corn syrup. Stir until melted and smooth. DO AHEAD: soaking syrup and ganache can be made 1 day ahead. Cover separately and store at room temperature.
For cake: Preheat oven to 350°F. Coat 15 x 10-inch sheet of parchment paper with nonstick spray. Line 15x10x1-inch baking sheet with prepared parchment (sides of baking sheet should not be coated).
Combine eggs, sugar, 2 teaspoons espresso powder, vanilla, and salt in large bowl. Using electric mixer, beat until egg mixture is thick and fluffy and falls in ribbons when beater is lifted, 6 to 7 minutes. Sift flour over in 3 additions, folding gently just to incorporate after each addition. Transfer 1 cup batter to small bowl; fold in butter. Fold batter from small bowl back into batter in large bowl just until combined; spread evenly on baking sheet.
Bake cake until tester inserted into center comes out clean and cake begins to pull away from pan, about 18 minutes. Cool cake in pan on rack 30 minutes.
Sift 2 tablespoons powdered sugar over cake. Cut around pan sides. Slide spatula under parchment to loosen from pan and invert cake onto work surface. Peel off parchment. Cut cake crosswise into 4 rectangles, each about 10 x 3 1/2 inches.
Place 1 cake rectangle on platter. Brush 2 tablespoons soaking syrup over. Spread 3 tablespoons ganache over, then 1/3 of espresso mousse (about 3/4 cup). Repeat 2 more times with cake, syrup, ganache, and mousse. Top with fourth cake layer; press lightly to adhere. Spread 2/3 cup plain mousse thinly over top and sides of torte to seal, then spread remaining mousse over top and sides. DO AHEAD: can be made 1 day ahead. Tent loosely with foil and chill.
Before serving, press praline pieces into mousse on all sides of torte. Mound white chocolate curls on top. Mix 4 tablespoons powdered sugar and 1 1/2 teaspoons espresso powder in small bowl. Sift over top of torte.