*If you would like to use store-bought caramel sauce, you will need 1 ¼ cup*
In a medium-sized heavy bottomed pot, heat water and sugar on HIGH heat and whisk just until the sugar is dissolved. It's important that you have high heat or you run the risk of seizing up the sugar, turning it into a solid mess.
Once sugar is dissolved, whisk gently and constantly until the melted sugar turns a dark amber color, this will take about 5 minutes or so. The darker the amber color, the more intense caramel flavor you will have.
After it turns dark amber, slowly and carefully pour in heavy cream. It will bubble up violently so you will need your whisk to whisk it very fast until it comes back down and it will start looking like caramel at this point. Remove from heat and stir in butter and vanilla. Allow to cool down a bit and thicken.
Slice apples very thin. Reserve a ⅓ cup of caramel and set it aside for drizzling after the bars are baked. Place apples in the pot of warm caramel and stir to coat. Allow them to sit and tenderize and soften slightly while you prepare your crust/crumble.