Cream butter and sugar, stir in nuts and graham cracker crumbs.
Line 9-inch cheesecake pan and press crumb mixture to the bottom, set aside.
Melt white chocolate in top of a double boiler over hot, not boiling, water.
Beat the cream cheese until light and fluffy. Stir into the melted chocolate.
Add the sour cream and sugar, stirring until the sugar is dissolved.
Beat the egg yolks until smooth, then stir them into the chocolate mixture.
Whip the egg whites until soft peaks form and fold them into the chocolate mixture.
Add the cognac.
Stir gently and pour into the crumb lined pan.
Bake 40 minutes.
Allow to cool, then chill for 8 hours.
Before serving, cover with whipped cream if desired and garnish with chocolate curls.