Preheat oven to 350 degrees, and grease a 9x13 baking dish. To make the crust, mix together the flour, butter, and brown sugar until crumbly. Press into your prepared baking dish and bake for 12-15 minutes, or until lightly browned. For the filling, beat the eggs, then add the corn syrup, sugar, butter, and vanilla with a wire whisk. Stir in the chocolate chips or chocolate chunks and the pecans. Pour over the crust and bake for 25-30 minutes, or until set. Cool completely, then refrigerate for about an hour before cutting into bars.