Preheat your oven to 350°F (175°C) and line an 8×8-inch baking dish with parchment paper, ensuring the sides are covered for easy removal later.

In a mixer, cream together softened butter, light brown sugar, and powdered sugar until pale and fluffy (about 3-5 minutes). Add in the egg and vanilla, mixing thoroughly.

Gradually fold in the all-purpose flour and ground cinnamon until a stiff dough forms. Split the dough in half.

Press half of the dough evenly into the bottom of the lined baking dish.

In a separate bowl, mix brown sugar, melted butter, flour, and ground cinnamon until it looks like a sandy paste. Spread over the bottom layer.

Roll out the remaining dough into a square and flip it over onto the filling, sealing the edges.

Bake for about 40 minutes or until golden brown. Let cool for 15 minutes before transferring to a wire rack.

Whisk icing ingredients until smooth and drizzle over cooled bars. Allow icing to set before slicing.