Preheat oven to 350°F and grease an 8 x 8 inch baking dish.In a large bowl, stir together the oats, brown sugar, baking powder, cinnamon, and salt.In a separate bowl, whisk together the almond milk, egg, and vanilla. Slowly add melted butter, whisking constantly.Pour milk mixture over the oat mixture and add apples. Stir until combined.Pour oats into the prepared baking dish and bake uncovered for 35-40 minutes, until top is golden brown and oats have set. Remove from the oven and cool 5 minutes. Serve warm.