Preheat oven to 325℉. Line a light metal 8x8" square baking dish with parchment paper, extending the sides of the parchment over the edges of the pan. Spray the parchment with cooking spray; set the pan aside.
In the bowl of a stand mixer, cream together the butter and granulated sugar with the paddle attachment until creamy, about 1 minute. Add in the confectioners' sugar and vanilla extract and beat on low-speed until just combined, about 1 minute, scraping down the bottom and sides of the bowl as needed. Lastly, add in the flour and mix on low speed until just combined.
Press half of the mixture into the prepared pan in an even layer. Take another piece of parchment and place it on the counter. Take the remaining dough and shape it onto the piece of parchment paper in a square shape roughly the same size as your pan; this will be your top layer. Set aside briefly.
For the filling ingredients: In a medium bowl, mix together the brown sugar, cinnamon, and flour. Add in the melted butter and stir together until the mixture becomes like wet sand. Sprinkle the filling mixture liberally onto the dough layer in the pan in an even layer.
Take your remaining dough on the piece of parchment and carefully peel the parchment off of the dough square (the dough may stick and/or rip a little; this is fine). Place the dough square on top of the filling and gently press the edges together to seal and repair any cracks that may have torn (if any).
Bake, uncovered, for 30-40 minutes or until light golden brown on the edges and the top appears set. Cool completely.
Once cooled, make your glaze: in a medium bowl, whisk together the confectioners' sugar, cinnamon, vanilla and heavy cream until smooth and pourable. Pour the glaze evenly over the bars and spread into an even layer. Allow the glaze to set completely before cutting into squares.
Bars can be stored at room temperature in an airtight container for up to 5 days.