Place butter, corn syrup and brown sugar in a heavy bottomed sauce pan; mix well and cook until butter and sugar have melted. Add sweetened condensed milk and cook over medium heat, stirring constantly, until caramel comes to a boil. Cook for an additional 2 minutes, continuing to stir constantly, then set aside.

Combine flour, brown sugar, oats and cinnamon in a bowl, pour melted butter over top and mix well until a crumble is formed. Pour caramel over third base. Top with crumble. Once the first 2 pans of squares are baked, bake this batch, also at 350°F (180°C) for 30 minutes.

Shortbread Base