• Move oven rack to middle position and heat oven to 350°F. Prepare a 9x9" baking dish with a foil sling, then grease the foil, making sure to grease the corners well.

Crust

• Pulse 1 cup whole wheat flour, 1/2 cup packed brown sugar, 1/4 cup pecan halves, 1/2 teaspoon salt, and 1/4 teaspoon baking powder together in a food processor until it looks like cornmeal, about 5 times. Add 6 Tablespoons unsalted butter, and pulse until the mixture looks like wet sand. Press the mixture into the prepared baking dish, and bake until it becomes fragrant, and begins to brown, about 20 minutes.

Filling

• While the crust is cooking, in a medium bowl whisk together 1/2 cup packed brown sugar, melted butter, 3 Tablespoons corn syrup, 2 Tablespoons pure maple syrup, 1 Tablespoon dark rum (if using), 2 teaspoons vanilla extract, and 1/4 teaspoon salt until the sugar dissolves. Add 1 large egg and whisk to combine.
• Spread 1 3/4 cup pecan halves over the crust, and pour filling over the pecans. Bake until the top is browned and beginning to crack, 22-24 minutes. Cool in the baking pan on a wire rack for 1 hour. Remove the foil sling from pan and peel foil from bars. Cut into squares.