Gather the Goods 6 tablespoons butter plus more for pan 1 granny smith apple, finely chopped 3 tablespoons brown sugar, divided + more for sprinkling 1/2 teaspoon cinnamon, divided 1 cup all purpose flour 1/2 cup freshly pulsed oat flour 1 1/2 teaspoons baking powder large pinch kosher salt 1 cup oat or almond milk…