Preheat oven to 350 degrees. Lightly spray two 9-inch cake pans with gluten-free non-stick cooking spray and line the bottom of each pan with a round of parchment paper cut to fit.
In a medium mixing bowl, whisk together the white or brown rice flour, sweet rice flour, potato starch, tapioca starch, xanthan gum, baking powder and salt.
In the bowl of an electric mixer, fitted with a paddle attachment, cream together the butter and sugar on medium speed until smooth and creamy, about 2 minutes. Turn the speed to low and add the eggs, one at a time, mixing well after each addition until fully incorporated and scraping down the sides of the mixing bowl after each addition. Add the vanilla and mix well.
With the mixer on low, add half the flour mixture, then the milk and finally the rest of the flour, mix just until incorporated. Remove the bowl from the mixer and scrape the sides and bottom of the mixing bowl well with a spatula. Divide the cake batter evenly between the two prepared pans and smooth the tops with a small spatula.
Bake cake layers 30 – 35 minutes or until the top springs back when touched lightly and a toothpick inserted in the center comes out clean. Run a dinner knife around the sides of the pans and allow the cake layers to cool for 30 minutes. Remove the cake layers from the pans and transfer to a wire rack to cool completely. Peel off the parchment paper.
While the cake cools, make the frosting.
In a heatproof bowl of a stand mixer, combine the egg whites, sugar and salt. Place the bowl over a saucepan containing an inch or two of simmering water; the bottom of the bowl should not touch the hot water. Whisk the mixture constantly either with a whisk or hand-held mixer, until the mixture feels warm to the touch, about 2 minutes. Test to make sure the sugar has completely melted by rubbing a little of it between your forefinger and thumb, the mixture should no longer feel gritty.
Attach the bowl to the mixer fitted with the whisk attachment and beat until the mixture is glossy, fluffy and completely cool, about 20 minutes. With the mixture on medium-low speed, add the butter a few tablespoons at a time mixing well after each addition. Once the butter had been fully incorporated, add the vanilla and melted chocolate and mix well. With a spatula, scrape down the sides and bottom of the bowl and stir the frosting for a minute to knock out the air bubbles; it should be completely smooth. If the frosting starts to curdle or separate after adding the butter, don’t worry, just keep beating until it is smooth again.
To assemble the cake, cut each cooled cake layer in half horizontally, using a serrated knife. Brush off all the crumbs. Lay the top half of one layer, top side down, on a cake platter. Tuck 3 or 4 strips of wax or parchment paper under the edges of the cake to keep your platter clean. Spread on a layer of frosting. Spread about 1/3 cup of jam on the cut side of the next layer and place it, jam side down on top of the frosting. Frost the top of that layer. Repeat with the remaining layers.
Using an offset spatula, spread a thin layer of frosting around the sides and on the top of the cake to catch any remaining crumbs. Let set for 5 minutes then frost the cake, swirling or smoothing the frosting as desired. Carefully remove the strips of wax or parchment paper.
For the cake I made for Wholesome!, I omitted the chocolate from the frosting, making a vanilla Buttercream, and melted 1 cup of chocolate chips with a tablespoon of shortening in the microwave until most of the chips were melted (about 1 minute) and then stirred vigorously until the chocolate was glassy and smooth. I let it cool a bit then topped the cake with it letting it drip down the sides of the cake.