Preheat the oven to 350 degrees. Grease a 9 x 13 inch pan.

In a mixing bowl, whisk together the flour, sugar, and salt. Cut in the chunks of butter with a pastry blender until fine crumbs begin to form. Press the mixture into the prepared pan. Bake for 15 minutes, or until crust is very lightly browned.

Combine brown sugar, Karo, and 1/2 cup butter in a saucepan. Bring to a boil over medium heat, stirring gently. Remove from heat. Put 1 cup of the hot mixture in a measuring cup. Slowly pour the hot mixture into the beaten eggs, whisking constantly. Add this to the remaining hot mixture. Stir in pecans and vanilla.

Pour the filling over the crust and bake for 30-35 minutes or until set.