Preheat your oven to 325°F. Line an 8-inch square baking dish with parchment paper, letting the paper extend over two sides for easy lifting later. Lightly spray with baking spray.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the softened butter and 1/2 cup brown sugar until smooth and creamy—about three minutes. Add the egg and mix to combine.

Add 3/4 cup confectioners' sugar, 2 tsp vanilla extract, and a pinch of salt, beating until just combined—roughly one minute. Gradually add 2 1/2 cups all-purpose flour, mixing until just incorporated.

Divide the dough in half. Firmly press one half into the base of your prepared pan.

Mix together 1 cup lightly packed brown sugar, 1 Tbsp ground cinnamon, and 2 Tbsp flour in a small bowl, then stir in 3 Tbsp melted butter until crumbly and spreadable.

Spread the filling over the dough in the pan.

Place the reserved dough square over the filling, pressing down gently to seal.

Bake for 30–40 minutes, or until the bars turn a light golden brown and the dough feels set to the touch. Let them cool completely in the pan.

Whisk together 1 cup confectioners' sugar, 1 1/2 tsp ground cinnamon, 1/2 tsp vanilla extract, a pinch of salt, and 3–4 Tbsp heavy cream (or milk) until smooth.

Drizzle the glaze over cooled bars, allow to set, then lift from the pan using the parchment and cut into squares.