6tablespoons(85g)butter, cut into 6 pieces, softened but still cool
1largeegg
1largeegg yolk
1teaspoonvanilla extract
⅓cupbuttermilk
Confectioners' sugar for dusting, optional
Instructions
For the topping, in a medium bowl, combine the sugars, cinnamon, salt and melted butter. Add the flour and stir with a rubber spatula or wooden spoon until the mixture is thick and smooth. Set aside to cool to room temperature, 10 to 15 minutes.
For the cake, adjust an oven rack to upper-middle position and preheat the oven to 325 degrees F. Cut a 16-inch length parchment paper or aluminum foil and fold lengthwise to 7-inch width. Spray an 8-inch square baking dish with nonstick cooking spray and fit the parchment into the dish, pushing it into the corners and up the sides; allowing the excess to hang over the edges.
In a large bowl (or the bowl of an electric mixer), mix the flour, sugar, baking soda, and salt. With the mixer running at low speed (electric or handheld), add the softened butter one piece at a time and continue beating until the mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes. Add the egg, egg yolk, vanilla, and buttermilk and beat on medium-high speed until light and fluffy, about 1 minute, scraping once if necessary.
Scrape the batter into the baking pan, spreading it into an even layer. Break apart the crumb topping that was set aside into large pea-sized pieces and spread the topping in an even layer over the batter. Bake the cake until the crumbs are golden and a toothpick inserted into the center of the cake comes out clean, 35 to 40 minutes. Cool on a wire rack at least 30 minutes. Remove the cake from the pan by lifting the parchment or foil overhang. Dust with confectioners' sugar, if desired, just before serving.
Notes
Flour: if all-purpose flour is substituted for the cake flour, the cake will be dry and tough. I usually don't have cake flour on hand so I use a substitution of cornstarch and all-purpose flour. I measure 2 tablespoons cornstarch into a 1-cup measure and fill the rest of the way with all-purpose flour. That ends up being 7/8 cup all-purpose flour and 1/8 cup cornstarch. Then I sift the mixture through a fine-mesh strainer onto a piece of waxed paper (repeating as necessary depending on how much I need - for this recipe there is a total of 3 cups of cake flour). Doubling: the recipe can also be doubled and baked in a 9X13-inch baking dish (increasing the baking time to about 42 minutes).