Preheat oven to 350 degrees. In a large bowl, using an electric mixer, beat butter, brown sugar, and salt on medium-high until light and fluffy, about 3 minutes. With mixer on low, add flour in three additions and beat until combined.

Line an 8-inch square baking dish with foil and spray with non-stick spray. Press dough evenly into baking dish. Bake until golden brown about 30 minutes. Let cool on a wire rack for 30 minutes then place in fridge or freezer to cool completely while you make the cookie dough.

In a large bowl, beat butter and granulated sugar with an electric mixer on medium speed until light and fluffy, 2 to 3 minutes. Mix in milk and vanilla. Stir in flour and salt and mix on low speed until incorporated. Gently spread cookie dough onto cooled shortbread, smoothing the top into an even layer.

Mix together the cinnamon and granulated sugar in a small bowl and sprinkle desired amount on top of bars. Lift foil out of pan. Cut bars into squares.

Refrigerate in an airtight container, will keep for up to 3 days.