FOR THE TOPPING: Pour the melted butter into an ungreased 9×2-inch pie dish or 9-inch round cake pan. Sprinkle the brown sugar evenly over the butter.
Pat the pineapple and cherries with paper towels until they’re very dry. (Wet fruit will make a soggy cake.) Arrange 6-7 pineapple rings and all the cherries on top of the brown sugar. Halve 3 pineapple rings and arrange them around the sides of the pan. Place the pan in the fridge to set the arrangement while you prepare the cake batter.
FOR THE CAKE: Preheat the oven to 375°F. In a medium bowl, whisk together the gluten-free flour, baking powder and salt.
In the bowl of a stand mixer, or using a hand mixer, beat together the sugar, egg whites, oil and vanilla until fully combined, scraping down the bottom and sides of the bowl as needed. Add the sour cream and beat until well-combined.
With the mixer running on low, add ½ of the dry ingredients, followed by ½ of the milk. Mix to combine. Add the remaining ½ dry ingredients and ½ milk, then mix until just combined.
Pour the batter on top of the pineapple arrangement. Bake for 40-45 minutes, until golden and a toothpick inserted into the center of the cake comes out clean.
Remove the cake from the oven and let cool for 20 minutes before turning it out onto a serving plate. (Do not let the cake cool for more than 20 minutes before turning it out.) Allow the cake to cool completely before serving