This grand finale features rye flour and autumn spices.
Makes 8 servings
2 1/4 cups sugar, divided
16 Tbsp. (2 sticks) unsalted butter, divided
2 medium sweet potatoes (about 1 pound), peeled
1 cup rye flour
1 cup all-purpose flour
1 1/2 tsp. baking powder
1 tsp. baking soda
1 1/2 tsp. kosher salt
1/2 tsp. nutmeg
1/4 tsp. cinnamon
1/4 tsp. allspice
1 cup plain whole milk yogurt
2 tsp. vanilla extract
Active time: 35 minutes
Total time: 1 1/4 hours
Combine 1 1/2 cups sugar and 1/2 cup water in a small, heavy-bottomed pot. Bring to a boil over high heat and cook, without stirring, until syrup is gently bubbling all across surface, 5 to 7 minutes. Remove from heat and stir in 8 Tbsp. butter in small pieces. Pour resulting caramel into a 9" cake pan.
Cut sweet potatoes into very thin slices using a mandoline or knife. Starting in center of cake pan, arrange sweet potatoes in a concentric circle, overlapping each piece over half of previous piece. Circle sweet potatoes out to edge of pan and then back toward center until there are 2 layers. Spoon 2 Tbsp. water on top.
Preheat oven to 350° and place a rack in the center. In a large bowl, stir together remaining 3/4 cup sugar, flours, baking powder, baking soda, salt, nutmeg, cinnamon, and allspice; set aside. Melt remaining 8 Tbsp. butter. In a separate bowl, stir together melted butter, yogurt, egg, and vanilla. Add butter mixture to flour mixture and stir to combine.
Spoon batter over arranged sweet potatoes, smoothing batter to edges of the pan. Bake, rotating pan halfway through, until cake is golden brown and springs back lightly to the touch, 50 to 55 minutes. Let cool 5 minutes, then place a plate on pan and invert, flipping cake onto plate. Let cool slightly before serving.