5 tablespoons unsalted butter, at room temperature
3/4 cup sugar
2 large eggs
2 tablespoons unsalted butter
1 cup powdered sugar
2 tablespoons maple syrup
2 tablespoons low-fat milk
1/3 cup chopped pecans, toasted
Preheat oven to 400°F and arrange bananas in a single layer in a baking dish; sprinkle with brown sugar and diced butter. Roast for 25 minutes, gently stirring mixture halfway through. Cool slightly.
Reduce oven temperature to 350°F and coat a 13 x 9-inch baking dish with nonstick spray. Whisk together flour, baking powder, baking soda, salt, cinnamon, allspice, and cardamom in a medium bowl. Combine buttermilk, banana, and vanilla in a small bowl. Beat butter and sugar in a large mixer bowl on medium speed until pale and fluffy. Add eggs and beat well. Reduce speed to low and add half of flour mixture to sugar mixture; beat just until combined. Add buttermilk mixture, then finish with remaining flour mixture.
Gently fold in roasted banana, then pour mixture into prepared baking dish and smooth top with an offset spatula. Bake 18 to 20 minutes, until golden and a toothpick inserted in the center comes out with a few crumbs attached. Remove from oven to cool completely in pan on a wire rack.
To make glaze, place butter in a small saucepan over medium heat. Cook, stirring constantly, until it foams, turns clear, and then turns a deep brown, about 6 minutes. Cool slightly. Sift powdered sugar into a small bowl; whisk in browned butter, maple syrup, and milk until smooth. Pour over cake and spread evenly with an offset spatula, then sprinkle with toasted pecans. Let glaze set before cutting into bars. Store leftovers in an airtight container at room temperature up to 2 days.