This cake is so good, there’ll be nothing but crumbs left in no time.
Adjust oven rack to upper-middle position and heat oven to 325 degrees. Make aluminum foil sling for 8-inch square metal baking pan. Spray foil lightly with vegetable oil spray.
For the crumb topping: In medium bowl, whisk together melted butter, brown sugar, cinnamon, ⅓ cup sugar, and ⅛ teaspoon salt. Add 1¾ cups flour and use rubber spatula to stir until mixture forms thick, uniform dough. Set aside.
For the cake: In bowl of stand mixer (or large bowl if using handheld mixer), combine baking soda, 1¼ cups flour, ½ cup sugar, and ¼ teaspoon salt. If using a stand mixer, lock bowl in place and attach paddle to stand mixer. Start mixer on low speed and mix until combined, about 5 seconds.
With mixer running, add softened butter, one piece at a time, and beat on low speed until mixture resembles coarse sand, about 1 minute. Stop mixer.
Add buttermilk, egg, egg yolk, and vanilla. Start mixer on low speed and beat until combined, about 20 seconds. Increase mixer speed to medium and beat until light and fluffy, about 2 minutes. Stop mixer and remove bowl from mixer, if using stand mixer.
Use rubber spatula to scrape down sides of bowl and stir in any remaining flour. Scrape batter into foil-lined baking pan and smooth top. Use your hands to break crumb topping dough into pea-size pieces. Sprinkle crumb topping in even layer over batter.
Place baking pan in oven. Bake until toothpick inserted in center of cake comes out clean, 35 to 45 minutes.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let cake cool in pan for 30 minutes.
Use foil to lift cake out of baking pan and transfer to cooling rack. Let cake cool completely, about 1 hour. Transfer cake to cutting board.
Even though these photos show our Raspberry Clafouti, the technique is the same. Here’s how to do it:
Remember: Protein in flour comes together during baking to form networks of gluten, which give structure to baked goods. More protein = more gluten = more structure. Compared to other flours, cake flour has the least amount of protein. And because we want a light and tender cake, we don’t want a lot of structure. That means cake flour to the rescue!