1cupold-fashioned rolled oatsnot quick oats or instant oats
⅔cupmelted buttercooled to room temperature
For the Apple Layer
2largeapplescored, pealed and sliced about ¼ inch thick
Preheat the oven to 350F degrees. Line an 8x8 or 9x9 inch square baking pan with aluminium foil and spray lightly with cooking spray or grease with butter.
In a large bowl whisk together the first 4 ingredients (from the flour to the salt). (Don't add in the extra 1 tablespoon flour & brown sugar listed at the end of the ingredient list - we'll add these in later). In a separate medium sized bowl whisk together the melted butter, ¾ cup brown sugar, vanilla and egg yolk. Add the butter mixture into the flour mixture, stirring everything together using a large wooden spoon or rubber spatula. The mixture will be like damp sand with some large clumps and some crumbly pieces. Reserve 1 heaping cup of the mixture, and press the remaining oatmeal mixture into the bottom of the prepared pan.
To make the apple mixture, toss together the sliced apples with the cinnamon, granulated sugar and cornstarch until the apples are evenly coated. Arrange the apples into an even layer over top of the bottom oatmeal layer.
In a small bowl mix the reserved 1 cup of oatmeal mixture with the extra 1 tablespoon of flour and brown sugar. Crumble the oatmeal mixture over top of the apple layer. Some of the crumbles will be large than others - and that's ok. Bake in the preheated oven for about 30 minutes, or until the mixture begins to brown on the top.
Allow to cool fully before cutting into bars. Bars can be warmed up before serving if you prefer.