Rhubarb Buckle

Cake: Preheat oven to 350 degrees, with rack in center position. Coat two 9-inch square cake pans with cooking spray, and line with parchment, leaving an overhang on 2 sides. Stir together rhubarb and 1 cup sugar; set aside to macerate.

Whisk together flour, baking powder, and salt. Beat together butter, remaining cup sugar, and the lemon zest until light and fluffy. Beat in eggs, 1 at a time, then beat in vanilla. Beat in flour mixture in 2 additions, alternating with sour cream, beginning and ending with flour mixture.

Crumb topping: Stir together flour, brown sugar, and salt. Add melted butter; stir to combine.

Divide batter between pans. Top with rhubarb mixture, and sprinkle with crumb topping. Bake until golden on top and cooked through, about 1 hour 5 minutes. Let cool completely in pans on wire racks, then lift cakes from pans using parchment. Remove parchment. Before serving, cut buckle into 2-inch squares.