Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 9x13-inch baking pan, or line with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: Prepare the Crust: In a large bowl, cream together the softened butter, salt, and granulated sugar until light and fluffy. Beat in the eggs one at a time, mixing well after each addition. Stir in the orange zest and vanilla extract.
Step 3: In a separate bowl, whisk together the flour and baking powder. Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
Step 4: Press about two-thirds of the dough evenly into the bottom of the prepared 9x13-inch pan. Reserve the remaining dough.
Step 5: Prepare the Cranberry Filling: In a medium saucepan, combine the cranberries, granulated sugar, orange juice, and orange zest. Bring to a simmer over medium heat, stirring occasionally, until the cranberries begin to burst and soften (about 5-7 minutes).
Step 6: In a small bowl, whisk the cornstarch with 2 tablespoons of water to create a slurry. Stir the slurry into the simmering cranberry mixture and cook, stirring constantly, until thickened (about 1-2 minutes). Remove from heat and let cool slightly.
Step 7: Pour the cooled cranberry filling evenly over the crust in the baking pan. Crumble the reserved one-third of the dough over the top of the cranberry layer.
Step 8: Bake for 35-40 minutes, or until the topping is lightly golden brown and the filling is bubbly. A toothpick inserted into the crust portion should come out clean. Allow the bars to cool completely in the pan on a wire rack before glazing.
Step 9: Prepare the Glaze: In a small bowl, whisk together the powdered sugar, orange juice (or milk), and vanilla extract until smooth. If the glaze is too thick, add more liquid a teaspoon at a time. If it's too thin, add more powdered sugar.
Step 10: Once the bars are completely cool, drizzle the glaze evenly over the top. Let the glaze set for about 15-20 minutes before slicing into squares and serving.