Chocolate Truffle Cake

Preheat oven to 350°F and butter a 9-inch springform pan. Make base:

In a small saucepan melt butter and stir in cocoa powder. Remove pan from heat and add brown sugar, stirring until dissolved. Stir in flour, walnuts, egg, and vanilla and spread batter evenly in springform pan. Bake base in middle of oven 10 minutes, or just until firm, and transfer to a rack to cool. Make filling:

Arrange walnuts in one layer on top of base. In a small heavy saucepan combine butter, brown sugar, and honey and cook over moderate heat, stirring occasionally, 8 to 10 minutes, or until a candy thermometer registers 280°F. Remove pan from heat and add cream, vanilla, and lemon juice, stirring until smooth. Cool mixture to room temperature and pour over walnuts, spreading evenly. Make ganache:

In a saucepan bring cream just to a boil. Finely chop chocolate. Put chocolate in a metal bowl and pour hot cream over it, stirring until smooth. Cool ganache to room temperature and beat with an electric mixer until it just holds soft peaks (do not overbeat or it will become grainy). Spread ganache evenly over filling. Chill cake, covered, at least 4 hours and up to 1 day. Run a thin knife around edge of cake and remove side of pan. With a large spatula transfer cake to a plate and let stand at room temperature 30 minutes before serving.