2. In a large mixing bowl, whisk together the almond flour, cornmeal, baking powder and salt. Set aside.
3. In the bowl of a stand mixer fitted with the whisk attachment, beat 1 cup of butter on high until light and fluffy, about 3 minutes. Stop occasionally to scrape down the sides, as needed. Gradually add the sugar, until the mixture is light in color and fully incorporated.
4. Add the eggs, one at a time, beating well after each addition followed by the buttermilk, ricotta, vanilla and lemon zest. Fold in the flour mixture and beat just until incorporated.
5. Carefully remove your hot skillet from the oven, and add remaining 1 tablespoon butter. Swirl until butter is melted and covers the whole surface. Carefully pour in cake batter, smoothing the top, and arranging the rhubarb in an even layer.
6. Bake for 45 minutes, or until the edges are dark golden brown and the center is set. Allow cake to cool completely in the pan on a wire rack before slicing. Serve warm or room temperature with whipped cream, if desired.