Preheat your oven to 325°F (163°C). Line an 8-inch square baking dish with parchment paper, ensuring the paper overhangs on two sides for easy removal. Lightly spray the parchment with baking spray.
In a stand mixer, cream together butter and brown sugar until smooth and creamy (about 3 minutes). Add the egg and mix until combined. Then add confectioners’ sugar, vanilla extract, and salt. Beat until combined, scraping down the bowl as needed.
Gradually add flour, mixing just until incorporated. Divide the dough in half. Press one half firmly into the prepared pan to form an even base.
On a separate sheet of parchment paper, shape the remaining dough into an 8×8-inch square and set aside.
For the filling, mix together brown sugar, cinnamon, and flour in a bowl. Stir in melted butter until well combined. Spread this filling evenly over the dough in the pan.
Place the reserved dough square over the filling, pressing gently to secure it. Bake for 30-40 minutes, or until the bars are golden brown and set.
While baking, prepare the glaze by whisking together confectioners’ sugar, cinnamon, vanilla extract, salt, and heavy cream. Adjust the consistency by adding more cream if necessary.
Once baked, let the bars cool in the pan for 10 minutes before lifting them out using the parchment paper. Allow them to cool completely on a wire rack.
Drizzle the glaze over the bars and spread evenly with an offset spatula. Let the glaze set before cutting the bars into squares.
