Preheat oven to 350°.
Line a 15″x 10″ jellyroll pan with aluminum foil. Lightly grease foil. Arrange graham crackers in a single layer in pan, slightly overlapping edges.
In a medium saucepan, combine brown sugar, butter, and cream. Bring to a boil over medium heat, stirring occasionally. Remove from heat. Stir in vanilla and pecans. Pour butter mixture over graham crackers and spread evenly.
Bake for 10-12 minutes, until lightly browned and bubbly. Sprinkle with salt. Slide foil from pan onto wire rack. Cool completely before breaking into bars.
To make a chocolate version, top bars with 1 cup semisweet chocolate chips after removing from oven. Let stand 3 minutes before spreading over bars. Add salt and continue as above.