Banana Chocolate Chip Blondies

Name: Banana Chocolate Chip Blondies
Ingredients: 1 tablespoon oil, for greasing; 1/3 cup (3 oz. / 85g) nut butter, such as cashew or peanut butter; 3 1/2 tablespoons unsalted butter; 2/3 cup (5 ½ oz. / 150g) firmly packed brown sugar; 1/4 teaspoon salt (if your nut butter is unsalted); 1 overripe banana, peeled (2 ¾ to 3 ½ oz. / 75 to 100g peeled weight) and mashed with a fork; 1 pinch baking powder; 1 teaspoon vanilla extract; 1/2 cup (2 ¾ oz. / 75g) whole-wheat pastry flour; 3/4 cup (3 ¼ oz. / 90g) oat flour or extra whole-wheat pastry flour; 1/3 cup (1 ¾ oz. / 50g) coarsely chopped dark chocolate (at least 70% cocoa solids)