• Preheat oven to 325°F. Line an 8×8-inch baking dish with aluminum foil, making sure it comes up the sides with some overhang. Spray with nonstick cooking spray. Set aside.

Crumb Topping

• To a medium bowl, add melted butter, granulated sugar, light brown sugar, cinnamon, and kosher salt. Stir to combine.
• Add cake flour and stir just until no streaks remain. Be careful not to overmix; the topping should be crumbly, not pasty. Set aside.

Cake

• In the bowl of a stand mixer fitted with the paddle attachment, add cake flour, granulated sugar, kosher salt, and baking soda. Mix on low speed to combine.
• Add butter pieces one at a time, mixing until the mixture looks like fine crumbs and there are no visible butter pieces, about 1 minute.
• Add buttermilk, egg, egg yolk, and vanilla extract. Beat on medium speed until the batter is light and fluffy, about 1 minute, scraping down the sides of the bowl as needed.
• Fold in ½ cup of mini chocolate chips, reserving the rest for the topping.
• Pour batter evenly into the prepared pan and smooth the top.
• Break the crumb topping into small chunks using your hands or a fork. Scatter it evenly over the batter, making sure to cover the top completely.
• Top with the remaining chocolate chips.
• Bake for 38 to 40 minutes, or until the crumb topping is golden and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.
• Let the cake cool in the pan on a wire rack for 30 minutes. Use the aluminum foil overhang to lift the cake out of the pan and cool completely.
• Dust with confectioners’ sugar just before serving.