Spread dough evenly into prepared pan and smooth the top. Bake for 18-22 minutes, until the top is slightly firm to touch, and a toothpick inserted into centre comes out with a few moist crumbs, not clean. Overbaking will make the bars dry. Allow to cool completely in pan.
To make the glaze: In a small bowl, combine powdered sugar, 1 tablespoon milk, and vanilla extract, and whisk until smooth. Add more milk as necessary until you get a thick yet pourable consistency. Drizzle over the bars using a spoon or a piping bag. Allow glaze to set, 15-30 minutes. For easy cutting, you can place it in the fridge for 2 hours until firm. Lift out of the pan and cut into squares. Serve at room temperature.