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 0.75 of mixture into an ungreased 20 cm 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.