For base, combine oats, sugar, nutmeats and butter, mixing until crumbly. Place in a shallow baking pan. Heat in preheated moderate oven (350 F.) 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 chocolate and vanilla. Add eggs, one at a time, beating well after each addition. Spread evenly over crumb base. Sprinkle with reserved crumb mixture. Freeze until firm. To serve, cut in pieces.