This is my very favorite gluten-free cake base. I use it all the time. I've made it into lovely layer cakes, Boston cream pie, perfect petit fours and every upside-down fruit cake that you can imagine. The key to its perfectly tender, buttery crumb is almond flour, my secret weapon for gluten-free baking.
For the oranges: Preheat the oven to 350 degrees F. Butter a 9-inch round cake pan and line the bottom with parchment paper; butter the parchment.
In a small saucepan, melt the butter over medium heat. Add the sugar, honey and salt and cook until the sugar has melted and turned golden brown, swirling the pan, 2 to 3 minutes. Remove the saucepan from the heat and carefully transfer the mixture to the prepared cake pan.
Cut the top and bottom off of each orange. Then cut the skin and pith off of each orange by slicing from top to bottom, following the curve of the fruit. Slice the oranges into 1/4-inch-thick rounds. Layer the rounds on top of the caramel in the cake pan. Set aside.
For the cake batter: In a medium bowl, whisk together the almond flour, cornstarch, baking powder and salt. In another medium bowl, beat the butter and sugar with an electric mixer on medium speed until fluffy, about 3 minutes. Beat in the egg yolks and vanilla. Add the almond flour mixture to the butter mixture and beat to combine. The batter will be stiff.
In a clean bowl with clean beaters, beat the egg whites with an electric mixer on high speed until they form medium-stiff peaks, about 2 minutes. Stir in about a third of the egg whites into the almond mixture to loosen it, then gently fold in the remaining whites. Transfer the batter to the pan with the oranges and smooth the top. Bake until the cake is golden brown and a toothpick inserted into the center comes out with moist crumbs attached, 40 to 45 minutes. Let the cake cool in the pan on a wire rack 10 minutes. Carefully flip the cake onto a serving plate. Serve warm or room temperature.