Ingredients
Vegetable-oil cooking spray, for cake pans
1 pound plus 10 ounces rhubarb, trimmed and cut 1/2 inch thick on the bias
2 cups sugar, divided
2 cups all-purpose flour
1 1/4 teaspoons baking powder
1 teaspoon coarse salt
1 1/2 sticks unsalted butter, softened
1 teaspoon finely grated lemon zest
3 large eggs
1 teaspoon pure vanilla extract
1/2 cup sour cream
1 cup all-purpose flour
1/4 cup light-brown sugar
1/4 teaspoon coarse salt
1/2 stick unsalted butter, melted
Preparation
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.
