Preheat oven to 350° Fahrenheit. Adjust oven racks as needed to allow cake pan to sit in center of oven. Spray 9-inch round cake pan with neutral-flavored cooking spray, making sure to cover bottom and sides of pan.
Pour ¼ cup unsalted butter (melted) into cake pan. Swirl and tilt pan to distribute butter across evenly across bottom of pan.
Sprinkle ⅔ cup packed brown sugar evenly over layer of melted butter.
Arrange 8-10 pineapple slices around bottom of pan, placing one ring in center of pan, then placing remaining pineapple rings around center ring. Pineapple slices should touch but not overlap.
Arrange 18 maraschino cherries around bottom of pan, placing 1 maraschino cherry in center of each pineapple slice, then using remaining cherries to fill in any spaces between pineapple slices.
Set cake pan aside.
For the Cake Batter
Add ¼ cup unsalted butter (at room temperature) and 1 cup granulated sugar to mixing bowl. Using stand mixer or hand mixer, beat ingredients together on low speed 1 minute, then increase speed to medium.
Continue mixing ingredients until mixture is pale in color and fluffy, approximately 3 to 5 minutes. Be careful not to overmix ingredients. Pause mixer as needed and scrape down sides of bowl with silicone spatula.
After creaming butter and sugar, pause mixer and add 1 teaspoon pure vanilla extract and 1 large egg. Mix ingredients together until fully incorporated into creamy mixture. Pause mixer as needed and scrape down sides of bowl with silicone spatula.
Pause mixer and add 1 ⅓ cups gluten-free all-purpose flour, 2 teaspoons fresh baking powder, and ½ teaspoon salt to mixing bowl. Mix ingredients together until fully incorporated, with no clumps of dry ingredients remaining. Pause mixer as needed and scrape down sides of bowl with silicone spatula.
Gradually add ⅓ cup full-fat sour cream and ⅔ cup milk of choice to mixing bowl, mixing constantly on medium-low speed until ingredients are incorporated and cake batter is thick and smooth. Pause mixer as needed and scrape down sides of bowl with silicone spatula.
To Bake the Cake
Carefully pour prepared cake batter into cake pan, covering pineapples and cherries completely. Use silicone spatula to smooth and spread cake batter evenly across pan.
Place cake pan in preheated oven. Bake cake 45 minutes.
After 45 minutes, check if cake is baked through by inserting toothpick vertically into center of cake then immediately removing toothpick. If toothpick is clean when removed from cake, remove cake pan from oven. If toothpick is covered in wet batter or cake crumbs, continue baking cake until toothpick is completely clean when removed.
When cake is baked through, carefully remove pan from oven and set aside. Let cake cool 10 to 15 minutes.
After 10 to 15 minutes, place cake plate face-down over top of cake pan. Place one hand on top of cake plate and place other hand under bottom of cake pan. In one motion, press cake plate and cake pan together while flipping cake plate and cake pan over. Cake plate should now be face-up with cake pan face-down on top of plate.
Tap bottom and sides of cake pan with hands or handle-end of butter knife, then slowly and carefully lift cake pan up and away from cake. Cake should release from pan easily, with pineapple-cherry layer on top side of cake.
Allow cake to cool on cake plate, uncovered, 10 minutes.
After 10 minutes, slice cake and serve as desired.
Recipe Notes
Measuring Flour: Use the spoon-and-level method to ensure you don't use too much flour.
Baking Powder: Most baking powder is gluten free, but to avoid any risk of cross-contamination, look for baking powder that's labelled "gluten free".