Preheat your oven to 350°F (175°C). Grease an 8x8-inch glass baking dish with butter or nonstick spray.
In a large bowl, whisk together the rolled oats, brown sugar, cinnamon, salt, and baking powder until well combined.
In a separate medium bowl, whisk together the milk, melted butter, beaten egg, and vanilla extract until smooth.
Pour the wet mixture into the dry ingredients and stir until just combined. Fold in the chopped nuts and dried fruit if using.
Pour the batter into the prepared baking dish and spread evenly. Bake for 25-30 minutes, until the top is golden brown and the center is set (a toothpick inserted should come out clean).
Remove from oven and let cool in the dish for 5 minutes. Cut into squares and serve warm, with a drizzle of maple syrup or a splash of milk if desired.