Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Make the crumb topping first. In a medium bowl, whisk 2 1/2 cups all-purpose flour, 1 cup packed brown sugar, 2 teaspoons cinnamon, and 1/2 teaspoon kosher salt. Stir in 1 cup (2 sticks) melted unsalted butter and 2 teaspoons vanilla until the mixture forms large, moist clumps. Squeeze some into big chunks. Refrigerate while you make the base.

Mix the cake batter. In a large bowl, beat 6 tablespoons softened unsalted butter with 1/2 cup granulated sugar until creamy, about 2 minutes. Add 1 large egg and 1 teaspoon vanilla; beat until fluffy.

Add dry and wet. In a separate bowl, whisk 1 1/2 cups all-purpose flour, 1 1/2 teaspoons baking powder, 1/4 teaspoon baking soda, and 1/2 teaspoon kosher salt. Add to the butter mixture in two additions, alternating with 3/4 cup whole milk (and 1 tablespoon neutral oil if using). Mix just until smooth. The batter will be thick but spreadable.

Spread the base. Scrape the batter into the prepared pan and smooth it into an even layer. Don’t worry if it seems thin; it will puff in the oven.

Top with crumbs. Sprinkle the chilled crumbs evenly over the batter, squeezing more in your hands to make big, bakery-style chunks. Use all the crumbs—they should form a generous layer.

Bake. Bake 28–35 minutes, until the cake layer is set and a toothpick inserted into the cake (not just the crumbs) comes out clean. The crumbs will look dry on top but feel slightly soft underneath when pressed.

Cool and finish. Let the pan cool on a rack for at least 30 minutes. Lift out using the parchment. Dust generously with powdered sugar right before serving.

Slice like a bakery. Cut into generous rectangles. The crumb layer should be thick, nubbly, and slightly sandy in texture.