Preheat your oven to 350°F (175°C).

Grease three 6-inch cake pans with oil and line with parchment paper, greasing the parchment too and lightly dusting with gluten-free flour.

In a large mixing bowl, combine cassava flour, brown rice flour, tapioca starch, powdered sugar, vanilla powder, light brown sugar, baking powder, baking soda, salt, and xanthan gum.

In a medium bowl, mix together strawberry rhubarb jam, oil, apple sauce, water, and vinegar until well combined.

Create a well in the center of your dry mixture and pour in the wet ingredients. Stir until just combined, leaving a few lumps.

Distribute the batter into the prepared pans and bake for about 35 minutes, checking for doneness with a toothpick.

Cool in the pans for 10 minutes, then transfer to a wire rack. Frost the layers with jam filling and decorate as desired.