FIRST, MAKE CRANBERRY SAUCE. If you're making cranberry sauce just for this, make the cranberry sauce in advance, so you can cool it before adding it to the cake. In a large saucepan, combine cranberries, 3/4 cup sugar, 3/4 water, 1 tsp. orange zest, 1/4 cup orange juice, pinch cinnamon, and pinch salt. Bring to a simmer over medium heat, stirring to break up the cranberries. Cook 8-10 min, or until thickened and reduced so you have about 2 cups of sauce. Cool sauce to room temperature or chill before using for the cake. (Cranberry sauce will keep fresh 3-4 days in the fridge, or can be frozen up to 2 months)
PREHEAT & PREP. To start, preheat the oven to 350 degrees F. Line a metal 8x8 pan with parchment paper and set aside.
MAKE THE CRUMB TOPPING. In a medium bowl, combine cooled melted butter, gluten-free flour, brown sugar, cinnamon, and salt. Stir with a fork until evenly combined. At first, it may look too wet. Don't panic! Just let it sit while you make the cake batter, and the flour will absorb liquid from the crumb.
MAKE ORANGE SUGAR. In the bowl of a stand mixer or a large mixing bowl, combine granulated sugar and orange zest. Rub the zest into the sugar with clean fingers until it's light orange and fragrant.
COMBINE WET INGREDIENTS. Add softened butter to the bowl and cream with a paddle attachment (or handheld mixer) on medium speed 2-3 minutes, until smooth and fluffy. Scrape down the sides of the bowl and add eggs one at a time, followed by the vanilla. Scrape down the sides of the bowl again.
ADD DRY INGREDIENTS. Add gluten-free flour, baking powder, baking soda, and salt and mix until JUST combined.
MIX IN SOUR CREAM. Scrape down the bowl one more time, then mix in the sour cream until the batter is smooth and even.
LAYER. Pour 1/2 of the cake batter into the prepared pan and smooth into an even layer. Top with 1 cup cranberry sauce and smooth the cranberry sauce into an even layer, leaving a slight border around the edge. Top cranberry sauce with the second half of the batter and smooth into an even layer, making sure to seal the edges well. Sprinkle the top of the cake with the crumb topping.
BAKE. Next, bake the coffee cake in preheated oven for 55-60 min, or until the crumb topping is golden brown and toothpick inserted in the center of the cake comes out clean with just a few moist crumbs (no wet batter!). The cake should spring back when gently pressed.
COOL, GARNISH & SLICE. Let the cake cool on a wire rack at least 30-45 min before cutting into slices. Garnish with a sprinkle of powdered sugar, then cut into 9 large squares, 12 medium rectangles, or 16 small squares and enjoy. Store leftover coffee cake in an airtight container or cover the pan tightly with plastic wrap or aluminum foil and store 1-2 days at room temperature or 3-4 days in the fridge.