Instructions

• Preheat the oven to 350°F (175°C). Generously butter a 13x9-inch baking pan with the butter marked "butter, for greasing the pan."
• In a medium bowl, whisk together 2½ cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, and ½ teaspoon salt. Set this dry mixture aside.
• In a large bowl or the bowl of a stand mixer, beat 10 tablespoons (1¼ sticks) unsalted butter (room temperature) and 1 cup granulated sugar until light and fluffy, about 3–4 minutes, scraping the sides of the bowl as needed.
• Add the 3 large eggs to the butter-sugar mixture one at a time, beating briefly after each addition until incorporated. Mix in 1 teaspoon vanilla extract.
• Add the reserved flour mixture to the wet ingredients, then add 1¼ cups sour cream. Beat just until combined and no large streaks of flour remain; do not overmix.
• Scrape the batter into the prepared 13x9-inch pan and spread into an even layer with a rubber spatula.
• Make the crumb topping: in a medium bowl, whisk together 3 cups all-purpose flour, 1 cup packed light brown sugar, 1 tablespoon ground cinnamon, and 1½ teaspoons coarse salt. Cut in 1¾ cups (3½ sticks) unsalted butter (room temperature) with a pastry blender, two forks, or a food processor (pulsing) until large, moist clumps form.
• Sprinkle the crumb topping evenly over the batter, covering the surface.
• Bake in the preheated oven until the top is golden brown and a toothpick inserted into the center comes out clean, 40–50 minutes. The center should feel slightly firm, not soft and soggy.
• Remove the pan from the oven and transfer to a wire rack. Cool until warm or at room temperature, then slice and serve.