Preheat your oven to 350°F (175°C). Grease an 8-inch square baking pan with butter or cooking spray and line the bottom with parchment paper.

In a medium mixing bowl, cream together softened unsalted butter and superfine sugar using an electric mixer on medium speed for about 3-4 minutes, until light and fluffy. Add pure vanilla extract and mix until well incorporated.

In another bowl, whisk together all-purpose flour and salt until combined. Gradually add this dry mixture to the creamed butter mixture, stirring gently with a spatula until just combined.

Divide the dough in half; press one half into the prepared pan evenly across the bottom. Cover the remaining half with plastic wrap and chill in the refrigerator for about 15 minutes.

Bake the pressed crust in the preheated oven for 10 minutes, or until slightly underbaked. Let it cool for a few minutes.

Mix together melted butter, brown sugar, cinnamon, and a splash of vanilla in a small bowl to prepare the cinnamon filling.

Spread the cinnamon filling evenly over the cooled crust. Take the remaining chilled dough and place it over the filling, covering it completely.

Bake for an additional 30 minutes, keeping a close watch for golden brown edges. The center may appear slightly underdone.

Remove from the oven and allow to cool completely in the pan at room temperature, about 1-2 hours.

Whisk together powdered sugar, a dash of vanilla, ground cinnamon, and water until smooth for the icing. Drizzle over the cooled bars.

Cut into squares and serve immediately or store in an airtight container for later enjoyment.