To make base, preheat oven to 350 degrees F. Line a 15 x 10 inch shallow baking pan (one inch deep) with foil, leaving a 2-inch overhang on the 2 short sides. Butter all 4 sides (but not bottom). Blend flour, brown sugar, and salt in a food processor, then add butter and pulse until mixture begins to form small (roughly pea-size) lumps. Sprinkle into baking pan, then press down firmly all over with a metal spatula to form an even layer. Bake in middle of oven until golden and firm to the touch, 15 to 17 minutes, then cool in pan on a rack.
1 cup unsalted butter
1-2/3 cups granulated sugar
1/4 cup light corn syrup
1/2 teaspoon salt
1-1/2 cups fresh or frozen cranberries
1 teaspoon vanilla
3 cups chopped, toasted pecans
To make topping, melt butter in a 3-quart heavy saucepan over moderate heat and stir in sugar, corn syrup, and salt. Boil over moderately high heat, stirring occasionally, until caramel registers 245 degrees F on thermometer, about 8 minutes. Carefully stir in cranberries, boil until caramel returns to 245 degrees. Remove from heat and stir in vanilla, then stir in pecans until well coated. Working quickly, spread caramel topping over base, using a fork to distribute nuts and berries evenly. (Tip: Don't overcook caramel as it gets very brittle.) Cool completely.
2 ounces bittersweet chocolate
To decorate, melt chocolate in top of a double boiler and drizzle over bars