Makes two 8-inch square cakes
1½ cups packed dark brown sugar
2/3 cup vegetable oil
1 large egg
1 cup buttermilk
2½ cups all-purpose flour
1 teaspoon baking soda
1 teaspoon salt
1 teaspoon pure vanilla extract
2 cups diced rhubarb, about ½-inch pieces
½ cup coarsely chopped walnuts
1 tablespoon unsalted butter, at room temperature
¼ cup granulated sugar
Preheat the oven to 350°F. Grease or butter two 8-inch square baking pans.
Mix the brown sugar and vegetable oil together in a large bowl until smooth. Add the egg, buttermilk, flour, baking soda, salt, and vanilla. Blend until moist. Fold in the rhubarb and walnuts. Divide the batter evenly between the pans. In a food processor, combine the butter and sugar until the mixture resembles wet sand.
Scatter evenly over the surface of the batter.
Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean. Let cool in pan for 5 minutes before turning out onto a baking rack.
Serve warm or let cool completely before serving.
