While the shortbread base chills, heat the brown sugar and ⅔ cup butter in a non-stick pan. Stir constantly until the butter and sugar melt to prevent the sugar from burning. Add the condensed milk, cinnamon, and vanilla, and bring the mixture to a boil on high heat as you continue to stir. You’ll need to keep stirring until the mixture thickens slightly — about 5 minutes. “Be very careful that none of the caramel splashes you, as it will be hot,” warns Dalton.
When the caramel has thickened, remove the shortbread from the fridge and pour it over the base in an even layer. Allow the caramel to cool for 20 minutes at room temperature. Once cooled, transfer the pan to the fridge or freezer for the caramel to set completely, about an hour.