Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Beat flour, brown sugar, pumpkin, 1/2 cup butter, eggs, cinnamon, vanilla extract, baking soda, nutmeg, ginger, and baking powder together in a large bowl using an electric mixer on medium speed until batter is evenly combined, about 2 minutes. Fold pecans into batter and spread into the prepared baking pan.
Bake in the preheated oven until edges are lightly browned, 20 to 25 minutes. Remove from oven and cool.
Beat 2 tablespoons butter and confectioners' sugar together in a bowl using an electric mixer until smooth; beat in orange juice and orange zest until frosting is light and spreadable. Spread frosting onto cooled bars.