Prep: Preheat oven to 350°F (175°C). Line an 8x8-inch baking dish with parchment paper.
Dough Base: Cream softened butter with brown and powdered sugars until fluffy. Beat in the egg and vanilla.
Combine: Fold in the flour and cinnamon until a stiff dough forms.
Bottom Layer: Press exactly half of the dough into the bottom of the prepared pan in an even layer.
Filling: In a small bowl, whisk together the filling ingredients (brown sugar, melted butter, flour, cinnamon). Spread evenly over the bottom dough layer.
Top Layer: Press the remaining dough into a square on parchment paper, then flip it onto the filling. Press edges gently to seal.
Bake: Bake for 40 minutes or until golden brown.
Icing: Whisk icing ingredients together. Drizzle over the bars once they have cooled for at least 15 minutes.
Serve: Allow icing to set before slicing into squares.
Notes
Soft Butter: Ensure butter is not melted for the dough, or the bars will be too greasy.
Sealing: Leave a tiny gap between the filling and the edge of the pan to help the dough layers fuse together.
Storage: Store in an airtight container for up to 5 days.