Prepare the Shortbread Crust: Preheat the oven to 350°F (175°C). Line an 8 x 8-inch baking pan with parchment paper, allowing some overhang for easy bar removal.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, granulated sugar, and brown sugar on medium speed until light and fluffy, about 3 minutes. Scrape down the sides of the bowl.
Add the egg and vanilla to the mixture, beating until well combined. Scrape down the bowl as needed.
In a separate bowl, whisk together the flour, baking powder, cinnamon, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until the dough pulls away from the sides of the bowl. Do not overmix.
Set aside roughly 2 cups of the dough for the crumble topping. Press the remaining dough firmly into the prepared baking pan, ensuring an even layer.
Bake the crust for 15 minutes or until the edges are lightly golden and the center is just set. Let it cool slightly while preparing the cranberry filling.
Make the Cranberry Filling: In a large mixing bowl, combine the cranberries, sugar, cornstarch, orange zest, orange juice, and vanilla. Pour the cranberry mixture over the pre-baked crust, spreading it evenly.
Add the Crumble Topping: Break up the reserved shortbread dough into small, flat pieces and place them randomly over the cranberry filling, leaving gaps for the cranberries to peek through.
Bake for 45 minutes, covering the pan with aluminum foil for the last 10-15 minutes to prevent over-browning.
Cool and Glaze: Allow the bars to cool completely on a wire rack before adding the glaze.
Prepare the Vanilla Glaze: In a small bowl, whisk together the powdered sugar, milk (or heavy cream), and vanilla extract until smooth. Adjust thickness as needed. Drizzle the glaze over the cooled bars.