Over medium heat, melt butter into a well seasoned, 9-10 inch cast iron skillet, and brush the sides with it. Add rhubarb, ¾ C sugar, lemon juice, 2tsp vanilla and pinch salt and stir for 2 minutes, just until combined and sugar has dissolved. Turn heat off. Do not continue to cook rhubarb.
In a medium bowl, whisk egg yolks with lemon zest, cardamon and ¼ C sugar, until creamy and sugar has dissolved. Set aside. In a small bowl, whisk almond flour, pinch salt, and baking powder until combined.
Add flour mixture to egg mixture and stir to incorporate. You will have a very thick heavy batter at this point.
In a stand mixer, whip egg whites on medium high speed, and once you see bubbles, add a pinch of salt and tsp vinegar . Whip until egg whites have formed soft peaks, then add remaining ¼ C sugar a little at a time whipping until medium stiff peaks form. Turn speed to down to medium low and begin spooning in the thick flour – egg yolk batter, a heaping spoonful at a time, into the egg whites, until it is all combined, scraping downs sides as necessary.
Pour batter over rhubarb in the cast iron skillet and place in a preheated oven, for 30 minutes. While baking, make sure to have ready, what you plan to flip the cake on to. At 30 minutes, check for doneness by inserting a toothpick or skewer into the middle of the cake. If it comes out clean, it is done. If not, bake 5 minutes longer, check again. Pull from the oven, and if making in a skillet, immediately place a cake platter or cake stand over the skillet, and invert, gently tapping the bottom of the skillet so it releases.
Serve with vanilla ice cream or on its own. The cake is especially good the next day.
I personally use a cast iron skillet for this recipe! I’ve tried it with a spring form pan- and no leaks- but readers are having issues with leaks. So I recommend a cast iron skillet. 😉