Grease an 8 or 9-inch round cake pan and line it with parchment paper.
In a large bowl, cream the sugar and butter in a bowl for about 3 minutes. Add the oat flour flour, almond flour, baking powder, salt, cinnamon, and eggs and beat well.
Spoon the batter into the prepared pan. Place the sliced plums skin side up on top of the batter, then sprinkle with additional sugar if you like. Rest the batter for up to 20 minutes, then preheat the oven to 350℉.
Place the torte in the oven and bake it for 60-70 minutes, until a tester comes out clean and the top is set. Remove from the oven and cool in the pan set on a wire rack for 30 minutes, then gently turn out to cool to room temperature.
Notes
Adapted from the classic New York Times Plum Torte recipe.A note on plums: You can use whatever plums you have on your trees or find in your local market. I like to slice the plums rather than halve them, as per the NYT, because the plums I have access to are pretty large. I needed about 6 plums to make this plum torte, but I think you can add more or less depending on how plummy you want your cake to be.To freeze, double-wrap the torte in foil, place it in a freezer-safe plastic bag, and seal it. Store the torte in the freezer for up to 3 months. Thaw it completely in the fridge before serving.Rest the batter - After you've assembled the cake, let the batter rest in the pan for 20 minutes or at least while the oven preheats. Gluten free flour always benefits from a short rest before baking so that the flour can hydrate fully.The plums will sink - Important! It is almost 100% certain that the plum slices will sink into the cake. So it won't have that NYT plum torte look where the plums are perched on the top of the cake. This is because gluten free flour is not as heavy as wheat flour. Flour substitutions - Sorghum flour by weight makes an ideal swap for the oat flour if you can't eat oats. Brown rice flour is another more economical option. For the almond flour, try cornmeal or another nut of choice. Or, use gluten free all-purpose flour.Sprinkle the top with cinnamon - For extra flavor, sprinkle the top of the cake with 1 teaspoon of ground cinnamon and 1 tablespoon of sugar.Bake as small cakes: Line a cupcake tin with liners, then fill each cup about ¾ full. Top the batter with berries or plums, sprinkle with cinnamon, then bake for about 40-45 minutes.