Beat the butter or margarine in a large mixing bowl with an electric mixer on medium to high speed for 30 seconds. Add brown sugar and beat till combined. Beat in egg yolk and vanilla till combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour with a wooden spoon. Spread dough in a lightly greased 30 cm pizza pan. Bake in a 180ºC F oven about 25 minutes or till golden. Sprinkle hot crust with 1 cup of the chocolate pieces. Let stand 1 to 2 minutes to soften. Spread chocolate over crust. Sprinkle with marshmallows and nuts. Bake 3 minutes more or till marshmallows are puffed. Cool in pan on a wire rack. Melt the remaining quarter cup chocolate pieces and shortening in a small saucepan over low heat, stirring constantly. Drizzle over cooled pizza. Cut into wedges. Makes dozen to 16 servings.