Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.

In a mixing bowl, cream together softened butter and sugar for about 3 minutes until light and fluffy. Add the egg and vanilla extract, mixing well.

Sift the all-purpose flour and cornstarch into the butter mixture. Mix gently until just combined.

Reserve about 2 cups of the dough for the crumble topping. Press the remaining dough evenly into the bottom of the prepared baking pan.

Bake the crust for around 15 minutes until lightly golden.

In a medium bowl, combine cranberries, sugar, cornstarch, orange zest, orange juice, and a splash of vanilla to prepare the filling.

Pour the cranberry filling over the baked crust, spreading it evenly.

Crumble the reserved dough generously over the cranberry filling.

Bake for approximately 45 minutes, covering with foil during the last 10-15 minutes to prevent over-browning.

Allow the bars to cool completely on a wire rack.

Whisk together powdered sugar, milk, and vanilla until smooth and drizzle over the cooled bars.

Lift the bars out of the pan using parchment overhang and cut into squares.