Grease an 8" baking pan with oil and line the bottom with a round of parchment paper.
Arrange the plums in concentric overlapping circles on top of the parchment paper and sprinkle with 1 tablespoon of the sugar.
Preheat the oven to 350°F.
In a medium bowl, sift together the chestnut flour, sweet rice flour, oat flour, xanthan gum, baking soda, baking powder and salt. Set aside.
In the bowl of a stand mixer, cream together the butter and remaining ¾ cups sugar over medium speed until light and fluffy, 3-4 minutes.
With the mixer on low-speed, add the eggs one at a time, mixing after each one.
Add the vanilla extract, mixing until just combined.
Keeping the speed on low, slowly add the dry ingredients in two batches, adding the milk in between, mixing until just combined.
Pour the batter over the plums and use a spatula to smooth it out.
Bake for 50-60 minutes, or until the center bounces back when touched and a toothpick inserted in the center comes out clean or with just a few moist crumbs. Place pan on a wire rack to cool completely (I left mine overnight).
When completely cool, use a thin knife to loosen the edges of the cake. With a plate or second wire rack on top, invert the cake, giving it a few taps as needed to release it from the pan. Carefully peel off the parchment paper.
Just before serving, brush the top with a little bit of honey to glisten up the fruit. Slice and serve as is, or with a scoop of vanilla ice cream.
Notes
*I used Flavor King pluots because they had the most flavor.**I've baked a version of this cake without the xanthan gum and, while it works, I think the texture of the xanthan gum cake is better - it's chewier and less crumbly, as a cake should be.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.