Combine flour, rolled oats, brown sugar and butter. Mix until well blended. Pat firmly into un-greased 13x9x2 inch baking pan. Sprinkle with pecans. To prepare Caramel Topping, combine butter and sugar in heavy saucepan. Cook over medium heat, stirring constantly until entire surface of mixture is boiling. Boil 1 minute. Remove from heat; add vanilla. Pour evenly over pecans and crust. Bake at 350 ?F until caramel is bubbly, about 15 to 18 minutes. Break chocolate into 1 inch chunks. Sprinkle evenly over caramel layer. Bake 1 minute longer to allow chocolate to melt. Swirl chocolate for a marbled effect. Cool slightly, then chill to set chocolate. Cut into bars.