Preheat your oven to 325℉. Line an 8-inch square baking dish with parchment paper, ensuring the paper extends over two sides for easy lifting. Lightly spray with baking spray.
In a stand mixer, cream together the butter and brown sugar until smooth and creamy, about 3 minutes. Add the egg and mix to combine.
Gradually add confectioners’ sugar, vanilla extract, and salt, beating until just combined, about 1 minute. Scrape down the sides of the bowl as needed. Gradually mix in the flour until just incorporated.
Divide the dough in half. Press one half firmly into the base of your prepared pan, ensuring an even layer. Shape the remaining dough into an 8×8 inch square on a separate piece of parchment paper and set aside.
For the filling, mix together the brown sugar, cinnamon, and flour in a small bowl. Add the melted butter and mix well.
Spread the filling evenly over the dough in the pan. Carefully place the reserved dough square over the filling and gently press down.
Bake for 30-40 minutes, or until the bars are light golden brown and set to touch. Allow the bars to cool completely.
While the bars cool, whisk together the confectioners’ sugar, cinnamon, vanilla, salt, and heavy cream for the glaze.
Once the bars are cool, lift them out of the pan using the parchment paper overhang. Drizzle the glaze over the bars and spread evenly with an offset spatula.
Allow the glaze to set, then cut into squares and enjoy!