Prepare the cake pans: Spray two 7" (or 8" - see notes below) round cake pans with cooking oil spray, or brush with melted butter). Line the bottoms with a circle of non-stick baking paper. Combine the 1 tablespoon of gluten free flour and 1 tablespoon of caster sugar in a small bowl. Pour it into the first cake pan, and swirl it around to coat the sides of the pan. Tip it out into the second pan, and repeat. Tip out any remaining flour/sugar mixture, and bang each pan upside down over the sink to remove any excess.
Sift together the gluten-free flour and xanthan gum, and whisk to combine well.
Bring 1" of water to a simmer in a saucepan that is just big enough to sit the bowl of a stand mixer, (or a large heatproof bowl, if using a handheld electric mixer) on top, ensuring that the bottom of the bowl doesn't touch the water. Place the whole eggs, egg yolk, caster sugar and vanilla into the bowl, and place the bowl over the water. Stir constantly until the egg mixture is warm and the sugar is mostly dissolved (it should be around 40°C if you have a candy thermometer). If you prefer to skip this step, make sure your eggs are warm before you start, by placing the eggs (in their shells) into a bowl of hot tap water for 10-15 minutes. Then crack them into the mixer bowl, add the sugar and vanilla, and continue with the next step.
Once the eggs are warm, whip them until they are thick and pale and make a thick ribbon when you lift the beaters. This takes about 10 minutes at medium-high speed in a stand mixer but may take a little longer with an electric hand-held beater.
Gently fold in the sifted dry ingredients in three-four additions. I like to use a large wire whisk to do this, as it doesn't pop as many of the air bubbles. A large metal spoon or silicone spatula will also do the trick.
Divide the batter evenly between the two prepared pans and then give them a tap on the benchtop to pop any large air bubbles.
Bake for 25-30 minutes, or until they're lightly golden and pulling away from the sides of the pan.
Allow to cool for 30 minutes in the pans, then slide a spatula around the edges to loosen them, tip the sponges out and place right-side-up on a wire rack to finish cooling.
Wrap with plastic wrap and store in an airtight container at room temperature for 1-2 days, or in the freezer for a month or two.
I like to use 7" round pans, as they then make layers that fit perfectly inside the trifle dish. If you only have 8" round cake pans, you can use those instead. The baking time will be roughly the same.Don't worry too much about accidentally over-baking these sponges, they will soak up the moisture from the coffee and cream in the tiramisu.I use my favourite gluten free flour blend for baking, which contains tapioca starch, brown rice flour and potato starch.