Preheat your oven to 350°F (175°C) and grease a cake pan.
In a mixing bowl, combine almond flour, coconut flour, baking powder, and baking soda.
In another bowl, cream together the sugar and butter until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the chopped rhubarb.
Pour the batter into the prepared cake pan, smoothing the top.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.