This chocolate chip crumb cake is unbelievably tender, loaded with chocolate chips and topped with the most amazing crumb topping!
NOTE: A number of readers have had problems with the volume of the crumb topping, so I am recommending that you cut it in half to avoid any issues.
Make the Crumb Topping: In a medium bowl, whisk the sugars, salt and butter to combine. Add the flour and stir with a rubber spatula or wooden spoon until the mixture resembles thick, cohesive dough; set aside to cool to room temperature, 10 to 15 minutes.
Make the Cake: Preheat oven to 325 degrees F. Spray an 8x4-inch loaf pan with nonstick cooking spray and line with parchment paper; allow excess to overhang edges of pan.
Using an electric mixer on low speed, mix the flour, sugar, baking soda and salt to combine. With the mixer continuing to ru n at low speed, add the butter one piece at a time; continue beating until the mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes. Add the egg, yolk, vanilla, and buttermilk; beat on medium-high speed until light and fluffy, about 1 minute, scraping once if necessary. Using a rubber spatula, fold the mini chocolate chips into the batter.
Transfer the batter to the loaf pan and spread into even layer. Break apart the crumb topping into large pea-sized pieces and spread in even layer over the batter, beginning with edges and then working toward center. Bake until the crumbs are golden and a wooden skewer inserted into center of cake comes out clean, 45 to 55 minutes (if the crumb topping appears to be browning too quickly, cover with foil for the remainder of the baking time). Immediately after taking the cake out of the oven, sprinkle with mini chocolate chips. Cool on a wire rack at least 30 minutes. Remove the cake from the pan by lifting parchment overhang. Dust with powdered sugar before serving. Leftovers can be stored at room temperature, wrapped in plastic wrap, for up to 2 days.
Do not substitute all-purpose flour for cake flour in this recipe, as it will result in a dry, tough cake.
Plain yogurt can be substituted for the buttermilk.