Preheat the oven to 325°F with a rack placed in the center of the oven. Butter a 9-inch cake pan.
In the bowl of an electric mixer, beat the butter and sugar together until fluffy, scraping down the sides periodically, for about 5 minutes.
Add the eggs, one at a time, beating until fully mixed with each addition.
Add the vanilla and mix completely.
In a small bowl, mix together the almond flour, all-purpose flour, and baking powder. Add to the batter, mixing until just combined. Do not over mix.
Stir in the rhubarb. Transfer the batter to the prepared cake pan and spread until the top is smooth.
Bake the cake for about 35 minutes, or until golden and a toothpick inserted into the center comes out clean. Remove from the oven and let it cool completely.