Preheat the oven to 350°F (180°C, or gas mark 4).

Have ready an 8-inch (20-cm) square baking dish.

To make the cookie base: In a large mixing bowl, combine the oats, flour, baking soda, baking powder, cinnamon, and salt.

In a medium mixing bowl, whisk together the milk, brown sugar, yogurt, and vanilla extract.

Fold the wet ingredients into the dry ingredients, mixing gently to avoid overmixing.

Spread the mixture evenly in the prepared baking dish.

Set aside.

To make the topping: In a medium mixing bowl, toss the apple chunks with the melted butter, brown sugar, and cinnamon until well coated.

Spread the apple mixture evenly over the cookie base in the baking dish.

Bake for 30 minutes, or until the apples are tender and the cookie base is set.

Remove from the oven and let cool on a wire rack for 15 minutes, still in the pan.

Transfer to the rack to cool completely before slicing into 16 squares.