Separate the yolks from the whites. Mix the yolks with sugar and lemon zest, and combine with dry ingredients. Whip the whites, and then combine all to create a batter.
Pour the almond flour batter over the rhubarb.
4 C cups rhubarb, cut into ½ inch cubes ( about 1 ¼ lb)
1 1/4 cup sugar -divided
4 tbsp butter
1 T fresh lemon juice
2 tsp vanilla extract
4 eggs, separated into 4 egg yolks and 4 egg whites, room temperature
zest from one lemon
¼ teaspoon cardamon
1 1/2 cups finely ground almond flour or meal
1 teaspoon baking powder
1 teaspoon white or cider vinegar
Powdered sugar for sprinkling
375 F Oven
Over medium heat, melt butter in a 9 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. (You can either bake the cake right in the same skillet, or transfer to a parchment lined and buttered 9 inch spring form pan.)
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 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 skillet ( or in a spring form pan) 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 a bit longer. Pull from the oven and immediately place a cake platter or cake stand over the skillet, and invert, gently tapping the bottom of the skillet so it releases. ( NOTE: If using a spring form pan, let cool before removing sides and flipping. ) Let it cool 10 minutes before serving. Serve with vanilla ice cream or on its own. The cake is especially good the next day.
Prep time: Cook time: Total time: Yield: 6-8