Preheat the oven to 350F degrees.
Line an 8x8 or 9x9 inch square baking pan with parchment paper.
In a large bowl whisk together the first 4 ingredients from the flour to salt
In a separate medium sized bowl whisk together the melted butter, 3/4 cup coconut (brown) sugar, vanilla and egg yolk.
Add the butter mixture into the flour mixture and mix everything together completely.
Set aside 1 heaping cup of the mixture.
Press the remaining oatmeal mixture into the bottom of the prepared pan.
For the apple mixture, mix together the sliced apples with the cinnamon, 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.
Bake for about 30 minutes or until it begins to brown on the top.
Cool completely before cutting.