Prepare your tin: Preheat the oven to 180°C (350°F). Line an 8-inch (20cm) square baking tin with parchment paper.
Make the cookie base:
In a bowl, whisk together melted butter and brown sugar until smooth.
Beat in the egg and vanilla.
Fold in the flour, baking powder and salt until combined.
Spread evenly into your lined tin.
Bake for 10 minutes to set slightly while you make the filling.
Cook the stewed apples:
In a small saucepan, melt the butter.
Add the diced apples, brown sugar, cinnamon and water.
Cook over medium heat for 5-7 minutes until the apples are softened and most of the liquid has evaporated.
Spread evenly over the warm cookie base.
Make the crumble topping:
In a bowl, combine flour, oats, sugar, and cinnamon.
Rub in the cold butter with your fingertips until the mixture looks like breadcrumbs with some small lumps.
Sprinkle evenly over the apple layer.
Bake: Return the tin to the oven and bake for 25-30 minutes, or until the crumble is golden brown and crisp.
Cool and slice: Leave the bars to cool fully in the tin before lifting out and cutting into squares.