brown sugar, firmly packed
For base, combine oats, sugar, nuts and butter, mixing until crumbly.
Place in a shallow baking pan.
Heat in preheated 350 degree oven about 10 minutes, stirring occasionally.
Firmly press 3/4 of mixture into an ungreased 8-inch square baking pan.
Set aside remaining crumb mixture for topping.
For filling, beat butter and sugar together until creamy.
Blend in melted and cooled chocolate and vanilla.
Add eggs, one at a time, beating well after each addition.
Spread evenly over crumb base.
Freeze until firm. To serve, cut in pieces.