Step 1

Preheat oven to 350°F. Melt butter in a 10-inch cast-iron skillet over medium. Transfer 1/2 cup melted butter to a small bowl; set aside. Stir brown sugar into remaining butter in skillet. Remove from heat. Scatter rhubarb pieces in a single layer over mixture in skillet.

Step 2

Place granulated sugar and orange zest in a large bowl. Using your fingers, rub zest into sugar until mixture is uniform in color, about 2 minutes. Add eggs to zest-sugar mixture; beat with an electric mixer on medium-high speed until pale and creamy, about 2 minutes. Whisk in sour cream, vanilla, and reserved 1/2 cup melted butter until combined. Sift together flour, baking powder, and salt in a medium bowl. Fold flour mixture into egg mixture just until combined. Spoon batter over rhubarb in skillet; smooth top using an offset spatula.

Step 3

Bake until a wooden pick inserted into center of cake comes out clean, 50 minutes to 1 hour. Let cake cool in skillet 15 minutes. Invert cake onto a serving platter; let cool slightly, about 30 minutes. Cut into wedges to serve. Cake may be stored, covered, in refrigerator up to 3 days (remove the cake from the fridge 30 minutes before serving).