Preheat & Prep. To start, preheat oven temperature to 350 degrees F. Line a metal 8x8 square baking pan with parchment paper.

Make The Crust Mixture. In a large bowl, combine cooled melted butter, brown sugar, granulated sugar, and vanilla. Stir to combine. Add gluten-free flour, gluten-free oats, baking soda, and salt and stir until clusters form and no dry patches remain. (The mixture will firm up as it sits!)

Divide The Crust Mixture. Set aside a very lightly packed 3/4 cup brown sugar oat crumble mixture for the top of the bars and pat the rest out into your prepared pan to form the bottom crust.

Make The Filling. In a medium saucepan, combine cranberries, sugar, cornstarch, orange zest, orange juice, and pinch of salt. Heat the mixture over medium heat, using the back of a spatula to gently crush the cranberries. Cook 15-20 minutes, stirring constantly, until the cranberry filling comes to a simmer and the sauce thickens. At first, the mixture will seem very dry, but as you continue cooking, the cranberries will soften and break down. Continue cooking so that the mixture is thick enough to coat the back of your spatula, and your spatula can easily make a visible trail in the bottom of the pan. (It'll look jam-like, like cranberry sauce.) Set filling aside to cool for 5 minutes.

Assemble The Bars. Pour the cranberry filling over the bottom crust and spread into an even layer. (An offset spatula makes this easy!) Sprinkle the reserved 3/4 cup of the crumble mixture over the top of the cranberry layer. (It won't completely cover the filling.)

Bake & Cool. Then, bake the bars in the preheated oven for 45-50 minutes, or until the topping is golden brown and the filling is bubbling around the edges. Cool 30 minutes on a wire rack, then transfer to the refrigerator to finish cooling completely. (This will make the bars easier to slice!)

Top With Glaze. If desired, combine powdered sugar, 1 Tablespoon milk, and vanilla extract in a liquid measuring cup or small bowl and whisk until smooth and just pourable, adding more milk if needed. Drizzle the glaze over the bars and let it set. (It should set quickly!)

Slice, Serve & Store. Use the parchment paper to lift the bars out of the pan and onto a cutting board. Carefully use a sharp knife to cut the bars into 12 rectangles and enjoy. Store leftover cranberry crumble bars in an airtight container in the refrigerator 3-4 days.