1. For praline, grease a cookie sheet. Cook the 1/3 cup sugar and the lemon juice in a saucepan over medium-low heat until sugar begins to melt (do not stir). Then cook, stirring occasionally, until sugar melts completely and turns deep golden. Stir in nuts; immediately pour onto sheet. Cool. Break up praline; process in food processor until finely ground.
2. For buttercream, combine water and 1/2 cup sugar in a small saucepan. Whisk over medium-high heat to dissolve sugar; boil until syrup registers 240 degrees F (soft ball stage) on candy thermometer.
3. Meanwhile, beat 3 egg whites and 1/8 teaspoon salt at medium speed in a mixer bowl, until foamy. Beat to stiff peaks. Slowly pour in boiling (240 degree F) sugar syrup. Beat until cool and thick. Beat in butter, a few pieces at a time, until blended. Add 3/4 teaspoon vanilla and beat until smooth and stiff.
4. Combine 1-1/2 cups of the buttercream and the ground praline. Stir melted chocolates into remaining buttercream.
5. For cake, heat oven to 425 degrees F. Grease a 15x10-1/2x1-inch jelly-roll pan. Line with waxed paper; grease and flour paper. Set aside.
6. Combine flour and 1/8 teaspoon salt. Beat the 2 eggs and 3 egg yolks in mixer bowl until thick. Beat in 1/2 cup sugar, 1 tablespoon at a time. Add the 1 teaspoon vanilla and beat until thick. Beat egg whites in clean mixer bowl until foamy. Gradually beat in remaining 1/4 cup sugar. Beat to soft peaks. Gently fold whites into egg yolk mixture just to combine. Sift flour mixture over egg mixture; gently fold mixtures until just combined. Spread batter in pan.
7. Bake cake until center springs back when lightly touched, 6 to 8 minutes. Sift 2 tablespoons confectioners' sugar over top. Immediately invert onto a clean towel. Remove pan and paper; sift remaining confectioners' sugar over top. Roll up cake from one long side with towel, jelly-roll fashion. Cool cake in towel.
8. Unroll cake. Set aside 1/3 cup praline butterceam; spread remainder on cake and reroll.
9. Thinly spread log with 1/3 cup chocolate buttercream (do not coat ends). Spoon remainder into pastry bag fitted with #98 (French star semicircle tip). Pipe parallel lines lengthwise to cover cake. Spoon remaining praline buttercream into pastry bag fitted with 1/4-inch round tip; pipe in circles to cover ends of cake.
10. Decorate cake with chocolate-dipped hazelnuts, curls, and currants. Makes 12 servings.