Make Shortbread Dough: In stand mixer with paddle attachment, cream butter and both sugars on medium speed for 3 minutes until light and fluffy. Add egg and vanilla, mixing until combined. Scrape down bowl.

Add Dry Ingredients: In separate bowl, whisk together flour, baking powder, cinnamon, and salt. Add to butter mixture and mix on low just until dough pulls away from sides of bowl. Do not overmix.

Reserve Dough: Set aside roughly 2 cups of dough (loosely packed) for crumble topping. Cover with plastic wrap and set aside.

Prepare Crust: Preheat oven to 350°F (175°C). Line 8x8-inch pan with parchment paper, leaving overhang on two sides. Press remaining dough evenly into pan using hands or flat bottom of measuring cup.

Pre-Bake Crust: Bake for 15 minutes until edges are slightly golden and center is just set. Do not overbake. Remove from oven and cool slightly.

Make Cranberry Filling: In large bowl, combine cranberries, sugar, cornstarch, orange zest, orange juice, and vanilla. Mix until cranberries are evenly coated. Pour over pre-baked crust and spread evenly.

Add Crumble Topping: Break reserved dough into smaller flat pieces with your hands. Place randomly on top of cranberry filling, leaving gaps for cranberries to show through. Do not cover entire surface.

Bake: Bake for 45 minutes. After 30-35 minutes, cover loosely with aluminum foil if topping is browning too quickly. Bars are done when filling is bubbling and topping is golden brown.

Cool: Remove from oven and place on wire rack. Cool completely to room temperature, at least 2 hours, before glazing.

Make Glaze: In medium bowl, whisk together powdered sugar, 2 tablespoons milk, and vanilla. Glaze should slowly run off whisk. Adjust with more milk or sugar as needed.

Glaze and Slice: Drizzle glaze over completely cooled bars. Let set for 30 minutes. Use parchment overhang to lift bars from pan. Cut into 16 squares with sharp knife, wiping clean between cuts.