Ingredients

• 4 cups all-purpose flour
• 1 teaspoon kosher salt
• 2 teaspoons baking soda
• 3 cups packed dark brown sugar
• 1/2 pound (2 sticks) unsalted butter, at room temperature
• 2 extra-large eggs
• 1 teaspoon pure vanilla extract
• 2 cups buttermilk
• 5 cups diced rhubarb (1/2-inch pieces)
• 1/2 cup granulated sugar
• 2 teaspoons ground cinnamon

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Instructions

1. Preheat the oven to 350°F. Lightly butter a 9 x 13 x 2-inch baking pan and line the bottom with parchment paper. Butter the parchment paper.
2. Make the cake; Stir the flour, salt, and baking soda in a large bowl and set aside.
3. Using a mixer set on medium speed, beat the brown sugar and butter in a large mixing bowl for 3 minutes, or until light and creamy. Beat in the eggs one at a time, beating well after each addition. Beat in the vanilla and buttermilk. Continue to beat for 2 minutes, or until smooth, occasionally scraping down the sides of the bowl.
4. Mix the flour mixture into the batter one-third at a time, beating briefly after each addition. Fold in the rhubarb until it is well distributed.
5. Pour the batter into the prepared baking pan. Use a rubber spatula to smooth it out to the edges of the pan. The batter will be very thick.
6. Make the topping: Stir together the granulated sugar and cinnamon in a small bowl. Sprinkle the topping evenly over the batter.
7. Bake the cake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean. Remove the cake from the oven and transfer to a wire rack. Cool for 1 hour before cutting the cake into squares to serve.