Sprinkle 3 cups pecans over a greased 15x10x1-in. baking pan.
Preheat oven to 350°. Cream butter and brown sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Gradually add flour to creamed mixture, beating well.
Spread batter into prepared pan. Combine candied fruit and remaining pecans. Spread fruit and pecans evenly over creamed mixture; press gently to help mixtures adhere. Bake until a toothpick inserted in center comes out clean, about 1 hour. Sprinkle rum over the top; cool completely in pan on a wire rack. Cut into bars. Store in an airtight container.
Test Kitchen tips
This holiday treat tastes like a cross between rum cake, fruitcake and pecan pie.
For a decadent twist, serve the bars over a swirl of creme anglaise or vanilla sauce.