Jaggery's mineral-infused sweetness with faint strawberry notes makes a good match for rhubarb. They combine well for this old-fashioned skillet cake. Soaking the whole-grain spelt flour overnight in yogurt softens the crumb of the cake.
The night before you plan to bake, beat the spelt with yogurt until loosely combined. Cover the bowl with tight-fitting plastic wrap lest it dry out, and set it in a warm spot in your kitchen overnight for 8 to 12 hours.
The next day, chop the rhubarb into ¼-inch dice. Toss it into a bowl and sprinkle ½ cup jaggery over the rhubarb. Allow the rhubarb to macerate in the jaggery for about an hour.
Preheat the oven to 350 F.
Melt 3 tablespoons of clarified butter or ghee over low heat in a well-seasoned 9-inch cast iron skillet. Toss the macerated rhubarb into the hot butter and saute about 2 minutes. Turn off the heat and prepare the cake batter.
In a large mixing bowl, beat the eggs with ¾ cup jaggery until smooth, then beat in the vanilla extract. Working ½ cup at a time, beat in the batter of yogurt-soaked spelt flour. In a separate bowl, whisk high-extraction flour with 1 teaspoon ground cinnamon, orange zest, baking soda and salt. Working ½ cup at a time, beat the dry ingredients into the wet ingredients until they form a smooth batter.
Prepare the jaggery crumb in a small to medium-sized mixing bowl. Beat the almond flour and jaggery together with cinnamon, then slowly beat in 3 tablespoons melted butter.
To assemble the cake, pour the cake batter into the skillet over the rhubarb. Sprinkle the jaggery crumb over the cake batter, and bake until a toothpick inserted into the cake's center comes out clean - about 1 hour. Allow to cool about 20 minutes before dipping into it with a spoon and scooping it out onto waiting plates.